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Daunting terrain and fanatical resistance threatened to take a horrific toll on GI invaders.

The operation for the occupation of Japan following the landing may be a very long, costly and arduous struggle on our part. The terrain, much of which I have visited several times, has left the impression on my memory of being one which would be susceptible to a last ditch defense such as has been made on Iwo Jima and Okinawa and which of course is very much larger than either of those two areas. According to my recollection it will be much more unfavorable with regard to tank maneuvering than either the Philippines or Germany.

– Secretary of War Henry Stimson to President Harry S. Truman, July 2, 1945

Safely removed more than half a century from the brutal World War II Pacific battles on Iwo Jima and Okinawa, critics of Harry S. Truman’s decision to use nuclear weapons against Japan in August 1945 confidently maintain that an invasion of the home islands – Kyushu in the south and central Honshu near Tokyo – would have resulted in far  fewer casualties than were generated by the atomic attacks against Hiroshima and Nagasaki. However, Truman’s critics fail to consider the horrifically lethal conditions GI invaders would have faced: unpredictable weather, daunting terrain, and millions of Japanese soldiers and civilians willing to fight to the death.


Beyond the facts that the Imperial Army was in somewhat better shape than is commonly assumed today and that the Japanese had correctly deduced the designated landing beaches and even the approximate times of the American invasions, a host of lethal tactical challenges faced GI invaders. For example, although the Japanese had never perfected central control and massed fire of their artillery, this fact would be largely irrelevant to the type of defense they were organizing. The months that the Imperial 16th Area Army on Kyushu had to wait for the American landings would not be spent with the soldiers and the island’s massive civilian population sitting idly, and their ability to dig in and pre-register their artillery cannot be casually dismissed.

To borrow a phrase from a later Asian war, each of the three initial Kyushu invasion areas was going to present Japanese defenders with a “target-rich environment” where artillery would methodically do its deadly work on a large number of U.S. Soldiers and Marines whose luck had run out. There was already ample evidence of artillery living up to its deadly reputation. In one notable instance on Okinawa, U.S. 10th Army commander Lieutenant General Simon B. Buckner was killed June 18, 1945, by Japanese artillery fire when the campaign was ostensibly in the mopping-up phase.

It has also been claimed that U.S. invasion troops need not have worried about Japanese cave defenses since combat experience had proved the effectiveness of the Americans’ self-propelled 8-inch and 155 mm howitzers against caves and bunkers as well as the caves’ vulnerability to direct fire from tanks. Yet the Japanese were also well aware of American cave-busting tactics and were arranging defensive positions accordingly from lessons learned on Okinawa and in the Philippines. This fact was not treated lightly in the Pacific as the Japanese had repeatedly demonstrated – on Okinawa, for example – that they could construct strongpoints that could not be bypassed and had to be reduced without benefit of any direct-fire weapons since no tanks – let alone lumbering self-propelled guns – could work their way in for an appropriate shot. Indeed, a U.S. I Corps intelligence officer who examined Kyushu’s terrain after the war found that the extensive rice fields were “held in by many stone terraces ranging in height from four to six feet [thus] precluding the off road movement by any type of military vehicle.”

Similarly, regarding the Japanese ability to defend against tanks, the Army and Marine armor veterans of the Pacific fighting would be amazed to learn from some of today’s historians that U.S. tanks would have had little to fear during the invasion. Despite the fact that Japan’s obsolescent 47 mm anti-tank guns “could penetrate the M-4 Sherman’s armor only in vulnerable spots at very close range” and that its older 37 mm guns were ineffective against Shermans, in reality, the Japanese through hard experience were becoming quite adept at tank killing.

During two actions on Okinawa, the Japanese knocked out 22 and 30 Shermans, respectively. In one of these fights, Fujio Takeda stopped four U.S. tanks with six 400-yard shots from his supposedly worthless 47 mm gun. As for the 37 mm, its use would depend on the terrain. Along likely axes of attack in valleys containing extensive rice fields, 37 mm guns would be positioned to fire into the highly vulnerable undersides of tanks rearing high in the air to cross the rice paddy dikes. In areas with irregular ground and vegetation, anti-tank fire would not be intended to destroy tanks but to immobilize them by blasting tracks and road wheels at short ranges to render the vehicles easier prey for the infantry suicide teams that had proved so effective on Okinawa.

Naval gunfire support has also been claimed as the American invaders’ trump card, since 25 U.S. Navy battleships and “big-gun” cruisers would be arrayed against Japan in the planned November 1945 invasion. The power of this force was unquestionably immense, prompting one awed author to state: “That the [Japanese] coast defense units could have survived the greatest pre-invasion bombardment in history to fight a tenacious, organized beach defense was highly doubtful.” As with many aspects of the planned invasion, however, perceived force ratios were not always what they seemed.

Half of these ships – 12 new “fast” battleships and battle cruisers screening the carrier task forces – were never slated to come within sight of Kyushu, although during the summer of 1945 some of them had bombarded steel mills along the Honshu coast in a failed effort to lure out Japanese aircraft. Additionally, the pre-Pearl Harbor battlewagons were to be divided up among four widely separated invasion zones, thereby diluting the effect of their shore bombardment fires.

Similar confident assertions about the decisiveness of planned naval bombardments had been made before; yet, even though every square inch of the much smaller islands of Iwo Jima and Okinawa had been well within the range of the U.S. Navy’s bombardment by 8-, 14-, and 16-inch guns during those campaigns, enough of the Japanese garrisons had survived to kill or wound 67,928 Soldiers and Marines.


What was Secretary Stimson getting at when he told President Truman that Japan’s terrain “will be much more unfavorable with regard to tank maneuvering than either the Philippines or Germany”? Stimson, a former artillery colonel during World War I, had conducted a leisurely tour of Honshu as a private citizen and visited twice in an official capacity. This presented him numerous occasions to cast his soldier eyes on the wide expanse of the Kanto Plain surrounding Tokyo. Stimson knew firsthand the daunting terrain GI invaders would face.

The U.S. Joint Chiefs of Staff set the date for the Kyushu invasion, called Operation Olympic, as X-Day, November 1, 1945, and for Honshu, Operation Coronet, as Y-Day, March 1, 1946. To reduce the number of casualties and lessen the chance of a stalemate, the launch of Coronet would await the arrival of two armored divisions from Europe. Attached to 8th Army, their mission was to sweep up Honshu’s Kanto Plain from the southernmost beachhead at Sagami Bay and cut off Tokyo before the seasonal spring rains, followed by the summer monsoons, turned it into vast pools of rice, muck and water, crisscrossed by elevated roads and dominated by rugged, well-defended foothills. East of Tokyo lay the invasion sites assigned to 1st Army.

Long before the British experienced the September 1944 Operation Market-Garden tragedy of trying to push XXX Corps’ 50,000 men up a single road through the Dutch lowlands to the “bridge too far” at Arnhem, U.S. planners were well aware of the costs that would be incurred if the Kanto Plain was not secured for mobile warfare and airfield construction prior to the wet season. Intensive hydrological and weather studies begun as early as 1943 made it clear that an invasion in early March 1946 offered the best mix of weather conditions for amphibious, mechanized ground, and tactical air operations, with movement becoming more difficult as the months progressed.

Weather in the Kanto Plain has always been unpredictable at that time of year. Indeed, the Tokyo area after the war experienced “sub-Arctic” conditions on the original March 1, 1946, invasion date, with several subsequent days of snowfall. March, the “transitional period between the dry winter months and wet summer months,” could well be “very dry or very wet,” but was thought not likely to present serious obstacles to tactical operations. April was a question mark – literally. In a staff study widely disseminated by U.S. invasion commander General Douglas MacArthur’s intelligence section, a very conspicuous question mark occupied only one of the 492 sections on the table-filled foldout containing weather data. Under the category “Rice Fields Flooded,” meteorological and geographical specialists refused to hazard either a “yes” or “no” answer as the extremely well-documented history of April weather in the Tokyo region demonstrated that there was too much seasonal variation in rainfall to accurately predict the condition of the ground.

Thus, with good luck, tolerably free movement across the Kanto Plain might be possible well into April. Unfortunately, this assumed that the snow runoff from the mountains would not be too severe, and that even during a “dry” March the Japanese would not intentionally flood the fields while waiting for the weather to lend its divine assistance sometime in April. Although subsequent postwar prisoner interrogations did not reveal any plans to deluge low-lying areas – interrogators did not ask, and Japanese prisoners did not comment on things about which they were not questioned – a quick American thrust up the Kanto Plain would not have been as speedy as planners desired.

First, none of the 5,000 vehicle bridges on the Kanto Plain (Stimson had personally traveled over many of them) were capable of carrying vehicles over 12 tons. Every tank, every self-propelled gun, and every prime mover would have to cross structures specifically erected for the event. Next, logistical considerations and the sequence of follow-up units would require that armored divisions not even land until Y Day+10. This would give defenders time to observe that the U.S. infantry’s tank support was severely hampered by “drained” fields that were almost never truly dry and to develop ways to make things worse for the invaders.

The danger was recognized by MacArthur’s intelligence shop, which, in “Summary of Weather Conditions, Tokyo Area – March,” carefully outlined the areas most susceptible to defensive flooding, while leavening its analyses with hopeful observations that the gooey belts at some locations were “narrow, mostly 100 to 200 yards wide” and “very narrow, from 50 to 300 yards wide.” Other areas that defied an upbeat assessment were simply described along the lines of “a 5- to 6-mile belt of large rice fields.”

The officers reading the intelligence group’s study needed no elaboration of the terrain’s tactical challenges to understand the statement: “During late spring, summer, and early fall, movement is, in general, restricted to roads, dikes, and embankments by floods and wet rice fields.” Likewise, the full-color chart “Effect of Rice Land, Natural, and Artificial Flooding on Cross-Country Movement” from the same document was also guaranteed to send chills up the back of any Soldier looking at it.

The principal effect of these materials was to reinforce, in clear, unambiguous terms, that the Kanto Plain must be seized by Y Day+45, or better yet, as close as possible to Y Day+30. A late start or loss of momentum on Honshu would leave American forces to fight their way up flood plains that were “dry” only during certain times of the year, but that could be suddenly inundated by the enemy. If the timetable slipped for either Olympic or Coronet (and virtually every major operation during the previous year had fallen far behind schedule), Soldiers and Marines on Honshu would risk fighting in terrain similar to that later encountered in Vietnam’s waterlogged Mekong Delta region – minus the helicopters to fly over the mess – where all movement was readily visible from even low terrain features and vulnerable convoys moved only on roads above sodden, impassable rice paddies.

This was a subject filled with immense implications because a maneuver problem of this scale could not be adequately addressed even if every bridging pontoon and associated piece of engineer equipment in the U.S. inventory could miraculously be sent to Kanto and be immediately available when and where it was needed.

The highly defensible terraced rice fields were a common feature on both islands and usually could not be easily bypassed because of the nature of their locations. The rice paddies stretch for miles along valley floors, and even when ostensibly dry they present formidable barriers to tracked movement and cannot be traversed by wheeled vehicles. Moreover, the sodden nature of most dikes and paddy floors are unsuitable for effective operation of devices like the hedgerow cutters eventually used by the Allies to “bust” through the bocage country in Normandy.

The rice paddies would have to be seized in a seemingly endless series of tedious, set-piece struggles through use of tactics similar to those employed in France’s bocage country before the appearance of the hedgerow cutter. Meanwhile, the armored elements fighting north up the roads past Tokyo in the west, and toward the capital in the east, would frequently find themselves limited to a one-tank front, as happened to the British when they were delayed reaching Arnhem by minimal German forces in the Dutch lowlands. U.S. attempts at flanking movements would be impossible or slowed to a crawl by a deadly combination of terrain and anti-tank weapons.

Japanese infantrymen were prepared to take on U.S. tanks with various personal anti-tank weapons, such as hollow-charge rifle grenades, the usually suicidal hand-placed satchel charges and a plethora of hand-operated hollow-charge mines. And when used in the proper tactical setting, traditional, if obsolescent, direct-fire weapons would become deadly tank killers during the invasion – especially on the Kanto Plain terrain. One of these, the Type 97 20 mm semi- or fully automatic anti-tank rifle, had thus far seen little use against American armor but had performed well against landing craft.

Even the comparatively thin frontal protection of the Sherman was too thick for the Type 97 to penetrate, but in the paddy fields it was a different story. At short range from expertly camouflaged positions, even a mediocre rifleman firing a semi-automatic to improve accuracy would be able to pump from two to a half-dozen 20 mm rounds into the half-inch belly armor of a Sherman as it reared up high over a dike. Once inside the tank, the rounds would smash into turret personnel, engine compartment and stored ammunition with catastrophic results.

The number of anti-tank rifles per Japanese division fluctuated according to the unit’s structure, but 18 was generally the minimum number. More robust formations, such as the Kwantung divisions sent to the home islands from Manchuria, fielded eight Type 97s per rifle company – some 72 per division. Likewise, the number of anti-tank guns ranged from 22 to 40, most of which were the more tactically flexible 47 mm. Nevertheless, great numbers of the 37 mm guns existed in artillery parks.

With Japan’s extensive preparations to use obsolete and obsolescent weapons in clever and unexpected ways to help repel GI invaders, it is certain that the Imperial Army would recognize that the dike structure presented unique opportunities for the effective employment of anti-tank weapons. Close coordination among American infantrymen and tankers could well keep losses from reaching intolerable levels; but there would be no quick armored thrusts on the Kanto Plain before the rainy season.


And then there is the matter of the Imperial Army’s long-range artillery. If there is one thing clear about the various operational schemes for the 1946 mechanized thrust out of the Sagami lodgment (hashed over in plans formulated as far back as the summer of 1944), it is that all appear to have been produced by planners who seemed blissfully unaware that a wall of mountains, the Kanto Sanchi, and their rugged foothills stretched north along the Americans’ left flank the entire distance of the planned 40-mile drive north.

Mount Fuji at its southern extremity is the feature’s most famous peak, and the mountain line comes complete with its own moat, the steep-banked Sagami River, which “forms a barrier to maneuver through or against the western foothills[’]” last 19 miles to the ocean. Broad expanses of the river’s lower regions could also be flooded to depths that would impede vehicle traffic; but even without assistance from the Imperial Japanese Army, “this river is deep and in [the] wet season floods to 1 mile wide.”

MacArthur’s intelligence section duly noted that “on the other hand, [the Sagami] also offers some protection to the west flank of a northward movement”; so perhaps the lack of interest was a byproduct of the military truism that a given piece of terrain may affect an enemy’s offensive operations just as much as it affects yours. Or perhaps it was a simple assumption that 8th Army’s assault would be conducted with such speed and violence that the mountains essentially would be irrelevant to the ground offensive. They weren’t.

There is no doubt that the lower Sagami was an effective block to Japanese ground operations launched from the foothills, but the principal threat from this area would have come not from enemy infantry but from Japanese artillery. Reinforcing the divisional artillery belonging to the mobile and coastal defense formations would be long-range guns placed well back into the foothills. A network of roads weaves its way through the heights, and while most were little better than trails by American standards, they were more than adequate for Japanese needs, principally because the Japanese had designed their artillery to be extremely compact and horse-mobile.

Although Japanese cannons were judged to be “not as rugged as those of comparable calibers in other armies,” they were perfect for the killing job at hand and received rave reviews in a U.S. War Department intelligence guide distributed down to platoon level: “Japanese artillery weapons exhibit the outstanding characteristic of lightness, in some cases without the sacrifice of range.” Not pleasant reading for a GI hitting the beach near Tokyo!

The entire expanse of the invasion area could be readily observed from anywhere along the foothills and mountains to their rear, with a clear view all the way to Tokyo Bay. U.S. forces could maintain reasonably effective smoke screens over the lodgment since the northern breeze averaged a workable 6 miles per hour that time of year, but with nearly all vehicle movement confined by terrain to known, preregistered targets, Japanese artillery literally would have been shooting fish in a barrel as American engineers and transportation elements struggled to clear blasted wrecks from the congested single-lane roads and restricted staging areas.

If well emplaced – and there is no reason to believe they would not be – these guns would be extraordinarily difficult to find and destroy by either air attack or counterbattery fire. The sky over the foothills would be far too “hot” for effective use of artillery spotting aircraft against the carefully camouflaged and protected guns. The long-range weapons themselves would not be diverted from their task by ground operations aimed at silencing them because, in terms of artillery, a variety of much shorter-range howitzers and mountain guns were available to defend the line of foothills to their front.

The dearth of American forces available for such an infantry-intensive task would be felt almost immediately as a brutal series of hill fights similar to that in Italy two years earlier (and in Korea five years in the future) was not anticipated by planners but would be thrust upon the Americans. Moreover, as U.S. forces clawed their way deeper into the plain, more and more of their left flank would be exposed to artillery in these foothills. At some point before Coronet, planners would certainly realize this; but as of August 1945, it had not yet been anticipated. Consequently, no significant number of troops had been allocated to this critical mission that would require a large and growing manpower commitment.


Stimson, the old artillery colonel during the brutal fighting of World War I, had personally viewed much of this ground, and Truman would not take lightly his appraisal of the targeted Japanese terrain. On the subject of casualties, the president did not need Stimson to explain to him what he meant by “an even more bitter finish fight than Germany” in his analysis in a June 18 conference with the president and the Joint Chiefs. All at the meeting knew it had cost roughly a million American all-causes casualties to defeat the Nazis, and that the number of American casualties was actually small when compared to those of the major allies. Moreover, Army Chief of Staff General George C. Marshall told President Truman the same thing at the meeting: that because of Japan’s terrain, “the problem would be much more difficult than it had been in Germany.”

Stimson’s warning to Truman that “we shall incur the losses incident to such a war” was equally clear. Stimson later recounted the meeting in a high-profile Harper’s Magazine article after Japan’s defeat. For any readers not understanding his assertion, he spelled it out: “We estimated that if we should be forced to carry this plan to its conclusion, the major fighting would not end until the latter part of 1946, at the earliest. I was informed that such operations might be expected to cost over a million casualties.”


 D.M. Giangreco served for more than 20 years as an editor for “Military Review,” published by the U.S. Army Command and General Staff College. He has written and lectured widely on national security matters and is an award-winning author of numerous articles and 12 books, including “The Soldier From Independence: A Military Biography of Harry Truman” (2009, Zenith Press) and “Hell to Pay: Operation Downfall and the Invasion of Japan, 1945-47” (Naval Institute Press, 2009).

Originally published in the July 2013 issue of Armchair General.