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In 1846 the Mexican War erupted along the Rio Grande River over the disputed Trans-Nueces region. The Mexican government insisted that the actual border was the Rio Nueces, 200 miles to the north. The intervening Trans-Nueces region was an uninhibited wasteland of no particular economic interest to either government, but the 1,800-mile-long Rio Grande stretched all the way to the Rocky Mountains – using it as the border could expand Texas into an empire.

The U.S. government’s 1845 legislation admitting Texas as a state did not define the territory it encompassed. Neither did the Mexican government define it when, at the same time, Mexico offered the Republic of Texas provisional recognition if Texas refused to become part of the United States.

Events developed slowly but steadily, and in August 1845 U.S. President James K. Polk sent an Army of Occupation under General Zachary Taylor to encamp at what is now Corpus Christi, just inside the southern boundary of the disputed Trans-Nueces territory. With later reinforcements, by spring of 1846 Taylor had nearly 4,000 soldiers, all regulars.

Meanwhile, in December 1845 Polk sent an emissary to the latest president of Mexico, General Jose Herrera, who had indicated a willingness to negotiate. If Herrera would sign over the Trans-Nueces region, Polk offered that the United States would forgive the $3 million in longstanding claims U.S. citizens had against the Mexican government for expropriations and losses they had suffered during the Mexican War of Independence. Additionally, Polk was offering up to $25 million to buy the rest of what was then northern Mexico. Total revenue for the debt-mired Mexican government had been only $20.6 million in 1844, its best year to that time (versus expenses of $31.3 million). Surely, the Mexican government would find such money irresistible?

But as Polk’s emissary, John Slidell, arrived in Mexico, Polk’s plans were leaked. The Mexican press went ballistic – who put up a for sale sign? Herrera therefore did not dare receive Slidell. To boost his damaged credibility, Herrera then ordered General Mariano Paredes in San Luis Potosi to march north to the disputed border. Paredes, who had recently helped Herrera overthrow General Antonio Lopez de Santa Anna and send him into exile, boasted a force of 8,000 soldiers – twice the number of men in Taylor’s army.

Paredes, indeed, did march. But he led his army south, to Mexico City, where he overthrew Herrera and installed himself as Mexico’s new president.

Dollar diplomacy having backfired, Polk told Taylor to move his army to the Rio Grande. Taylor sortied on March 8, 1846. On March 28, the force arrived at the Rio Grande opposite Matamoros, having made a side trip to seize a port for receiving supplies by sea at Point Isabel, 23 miles northeast of Matamoros on the Gulf of Mexico coast.

Taylor’s army encamped on a duly leased farm field directly across the Rio Grande from Matamoros and began to construct an earthen fort that was intended to communicate a permanent U.S. presence to Mexican authorities. Built in a vague star shape with six corner bastions, the fort featured walls that were 9.5 feet high by 15 feet thick and its 800-yard perimeter was fronted by a ditch 8.5 feet deep. Thousands of cubic yards of dirt had to be excavated with hand tools, causing the work to continue night and day.

Although Matamoros had a garrison of about 3,000 Mexican soldiers, the two sides did not exchange gunfire. The U.S. force wanted to finish the fort, and the Mexican force suffered from a command vacuum.


Paredes, following his coup in Mexico City, had dismissed the commander of the Matamoros area, General Mariano Arista (a future president of Mexico), for not supporting his takeover. He then had General Pedro de Ampudia scrape together about 2,200 second-line troops left in San Luis Potosi and head north. Ampudia’s arrival in Matamoros brought the garrison to 5,200 soldiers and 26 cannon. However, Ampudia’s appointment triggered a political crisis, as his well-earned reputation for brutality against Mexican civilians caused the locals to demand Arista be restored to his position.

Paredes eventually complied, but then nothing happened while the seething Ampudia awaited Arista’s return. Meanwhile, the area’s population was so restive that the Mexican commanders dared not use the usual IOUs or expropriations to gather supplies. Being broke, the Mexican forces faced chronic shortages.

When Arista returned, he decided to act immediately while he still held a numerical superiority over the American force, surmising correctly that Taylor would promptly ask for reinforcements. Thus, on April 23 Arista sent a mounted force of 1,600 troopers across the Rio Grande to get between Taylor and Point Isabel.

Hearing of the Mexican movement, Taylor sent scouting patrols upstream and downstream on April 24. The following day, Thornton, leading the upstream patrol, was ambushed. Taylor learned of this on April 26 when two of Thornton’s wounded dragoons were released to the Americans because Arista claimed he had no way to care for them.

Before the Mexican mounted force could do much more damage, Arista sent the men to a point 13 miles downstream from Matamoros to cover the planned crossing of his main infantry force, which began April 30.

The next afternoon, Taylor learned that Arista’s infantry had begun crossing the river. Within two hours Taylor had his army on the road back to Point Isabel, hoping to secure it against the Mexican threat and then return to the fort with more supplies. At the hastily finished fort, Taylor left an infantry regiment and three artillery batteries, totaling about 500 soldiers. He reached Point Isabel around noon the next day, May 2. At about that same time, Arista finished crossing the river and set out toward Point Isabel, leaving a force under Ampudia to watch the fort.

On May 3, Arista reached the Palo Alto area and found that he had missed Taylor, who had already passed through on his way to Point Isabel. Arista decided to block Taylor’s return route to the fort, encamping his army at a place where he could cover the Point Isabel road and another nearby “fair weather road.”

Also on May 3, the Mexican artillery bombardment of the fort began, continuing during daylight hours for the next six days. Taylor’s men at Point Isabel could clearly hear the bombardment, adding a sense of urgency to their task of strengthening Point Isabel’s defenses and returning to the fort as soon as possible.

By the afternoon of May 7, Taylor’s soldiers had fortified the port and filled 270 supply wagons. Then they headed back to the fort, traveling seven miles before camping.


Around noon the next day, May 8, Taylor’s force arrived at the watering hole near Palo Alto (or “Tall Timber,” so called because its trees were the first encountered when approaching from the coast.) The flat prairie provided no protective cover, while thick, knee-high cord grass, scattered ponds and muddy ground restricted off-road movement. Random clumps of brush offered only minimal concealment, but denser thickets did limit visibility to the west and south. Taylor found Arista’s force deployed perpendicular to the road to his front, as the Mexican army had marched from its campground to the southeast.  

The U.S. force consisted of about 2,300 soldiers, including a young 4th Infantry Regiment lieutenant named Ulysses S. Grant. Taylor formed the men on a line about 1,000 yards long, with the wagon train gathered in the rear. Two 18-pounder cannon were in the middle of the line, along with two batteries of four six-pounders each. These six-pounder batteries, known as “flying artillery,” were trained by Major Samuel Ringgold. (See Combat, November 2013 ACG.) Employing well-rehearsed drills, they would dash up to a point near the enemy line just beyond musket range, fire several rounds into the opposing ranks and then dash away. The speed, maneuverability, firepower and accuracy of the “flying artillery” batteries gave American forces an overwhelming tactical advantage in combat.

Arista’s force of about 3,700 soldiers was posted on a mile-long line, with the largest cavalry force on the west flank. Two eight-pounder guns and six four-pounders were scattered along the Mexican line.

The Mexican artillery began firing about 2:30 p.m. when the U.S. force was approximately 700 yards away. The rounds, however, were largely ineffectual since Taylor’s men could easily see – and thus avoid – the solid-shot cannon balls as they hit the ground and then bounced along. When the superior U.S. artillery responded, firing bursting shell and canister as well as some solid shot, the carnage inflicted on the Mexican line was ghastly.

This was the theme for the rest of the day, as the excellent U.S. artillery dominated the field and kept Arista from capitalizing on his numerical advantage in troops. Arista tried to send cavalry around the west flank, but the Mexican horsemen literally bogged down in the muddy, broken terrain and were easily driven off. Combat operations there were halted between about 4 p.m. and 5 p.m. due to grass fires. The greatest loss to Taylor’s force was the death of Major Ringgold, who fell mortally wounded.

Arista then launched an attack on the east flank, but it too was unsuccessful and his retreating soldiers disrupted the Mexican line. With the sun setting, Arista decided to pull his force to the south behind a chaparral thicket, and the fighting ended with Taylor’s victorious army holding the battlefield.

The superbly trained, aggressively led U.S. artillery had fired about 3,000 rounds of bursting shell, canister and solid shot for devastating effect. On the other hand, Arista’s overmatched gunners had fired only about 600 rounds, all solid shot, for little effect. Taylor’s casualties amounted to six dead and 40 wounded. Mexican casualties totaled 102 killed and 150 wounded or missing. Generally, the Mexican soldiers had bravely behaved as if they were on parade, yelling, “Viva!” and closing into tight, massed formations where, as witnesses noted, American artillery rounds tore “lanes” and “vistas” through the packed Mexican ranks.


During the night of May 8-9, Arista decided to put some distance between his men and Taylor’s force. On the morning of May 9, he led the Mexican army south on the Matamoros road six miles to its crossing of Resaca de la Palma. The terrain beyond the road corridor was an increasingly dense chaparral thicket that was difficult for troops in formation to negotiate. Arista’s force was joined by Ampudia’s troops, who had left the siege of the fort and come north on the Matamoros road.

Arista deployed his force behind the vegetation-covered banks of the wide, dry resaca (former riverbed), where they had cover from the U.S. artillery that had done such damage to his army at Palo Alto. (See Battle of Resaca de la Palma map.) The Mexican line extended about 1,000 yards on either side of the road, with seven cannon covering the crossing. Arista assumed that by the time Taylor’s force arrived and deployed, it would be too late to fight that day.

Taylor spent several hours at Palo Alto on May 9 fortifying his wagon train, and then he followed Arista, making contact about 2 p.m. To Arista’s surprise, Taylor immediately attacked, deploying infantry units to the right and left as they arrived. Lieutenant Charles May led a mounted charge against Arista’s artillery, and back-and-forth fighting erupted at the crossing.

The U.S. infantry units that deployed to the west found that the road closely paralleled the resaca, and they were soon in contact with its defenders. Small unit melees, including hand-to-hand combat, broke out. Later U.S. arrivals found a cow path that led them across the resaca beyond the Mexican west flank. They pressed their advantage and the entire Mexican position soon collapsed. Arista’s force withdrew in disorder three miles south across the Rio Grande River.

Taylor’s second victory in two days cost his army 45 killed and 98 wounded. Mexican casualties were 154 killed, 205 wounded and 156 missing (reports claim that many Mexican soldiers drowned during their panicked crossing of the Rio Grande).

Upon leading the army on to the fort, Taylor arrived there that evening to find that its commander, Major Jacob Brown, had died earlier that day after being severely wounded by a Mexican artillery shell on May 6. The Mexican bombardment had inflicted only one other fatality among the fort’s defenders. Taylor named the earthwork Fort Brown.

The U.S. troop reinforcements Taylor had requested earlier arrived over the next days – along with heavy rain. On May 17, Arista abandoned Matamoros and retreated southwest to Linares, leaving behind his artillery and about 400 wounded Mexican soldiers. Arista began his hastily organized retreat with approximately 4,000 men and arrived 11 days later with only 2,638.


On May 11, two days after the Battle of Resaca de la Palma, President Polk received word of Thornton’s April 25 ambush. It was exactly the kind of incident he wanted to justify a U.S. war against Mexico. He immediately asked Congress to declare war, saying that the Mexican army “has passed the boundary of the United States, has invaded our territory and shed American blood upon the American soil.” Congress complied, declaring war against Mexico on May 13 and authorizing a six-fold expansion of the U.S. Army.

Of course, by then Taylor’s victories at Palo Alto and Resaca de la Palma had already won the disputed Trans-Nueces region, establishing the Rio Grande River as the boundary by American force of arms. But Polk had a much larger vision for what his new war with Mexico could achieve. The body of water he now had his sights set on was the Pacific Ocean, not the Rio Grande River.

Taylor pressed on to Monterrey and then to Saltillo in northern Mexico, and other U.S. military expeditions would seize all of the Mexican territory that Polk had offered to buy – but the Mexican government remained unresponsive.

So, in 1847 U.S. planners prepared an expedition under General Winfield Scott to land at Vera Cruz on the Gulf of Mexico coast and then march overland to Mexico City, where it was hoped Scott could dictate terms. Naively believing that Santa Anna could negotiate a quick Mexican surrender, American authorities permitted the exiled politician-general to return to Mexico through the U.S. blockade.

However, the crowd-pleasing but erratic Santa Anna reasserted control in Mexico and then by supreme effort concentrated an army of more than 20,000 soldiers against Taylor, whose best troops had been sent to join Scott’s expedition. After a 240-mile advance across the desert in winter, and after losing the February 23 Battle of Buena Vista/Angostura (despite a 3-to-1 advantage), followed by a retreat across the same desert, Santa Anna had lost half his army. He scratched together a force to oppose Scott’s advance, but between losing more battles, violating truces and soliciting bribes from Scott, Santa Anna eventually lost Mexico City and resigned.

Thornton had been freed in a prisoner exchange and he marched with Scott; his bad luck, however, was unchanged. Thornton was killed by artillery fire on August 18, 1847, while leading another reconnaissance just before the Battle of Contreras near Mexico City.

Santa Anna’s successors ratified the Treaty of Guadalupe Hidalgo on February 2, 1848. The treaty not only officially recognized the border as the Rio Grande but also added to the United States vast territory that is today California, New Mexico, Nevada, and Utah, most of Arizona and Colorado, and slivers of Wyoming, Kansas and Oklahoma. The United States gave Mexico $15 million and covered the longstanding prewar claims.


At almost the same time as the treaty ratification, word of a huge gold strike began filtering out of California and the Trans-Nueces was forgotten. It remains forgotten. For instance, the Rio Grande Valley is the only metropolitan region in the contiguous United States not reached by the Interstate Highway System. Sadly, due to upstream irrigation, the river now has about one-fifth of the flow it had in 1846.

The Fort Brown earthwork was abandoned shortly after Taylor occupied Matamoros following Arista’s departure. Most of it was obliterated by later Rio Grande levee construction. The fort’s southwest corner survives as a serpentine, brush-covered mound in the northwest corner of the Fort Brown Memorial Golf Course (it is actually outside the U.S. border fence). Fort Brown the military post was later established a few hundred yards to the north of the earthwork, and the city of Brownsville grew up beside it. Decommissioned in 1946, it is now a college campus.

At the site of the Battle of Resaca de la Palma, there is now a modern bridge where Parades Line Road crosses the resaca, which has been dredged and flooded to serve the area’s municipal water system. The battlefield northwest of the bridge, where most of the fighting took place, is lost under suburban development. A largely vacant 35- acre tract of land just northeast of the bridge, formerly a polo field, was finally acquired by the U.S. National Park Service as the Resaca de la Palma Battlefield in 2011.

Aside from improved drainage, the Palo Alto battlefield has changed little. The Palo Alto Battlefield National Historical Park was established in 1978 and now owns about 1,600 acres of the battlefield, mostly in the area the Mexican army defended.

As for the Thornton ambush site, in late 1847 freshman congressman Abraham Lincoln tried to force the government to prove that the site had actually been American soil. He was ignored – and that’s a pity, since proof would have involved identifying the exact site, which we can’t do today. A memorial to the Thornton incident was erected in 1936 marking a candidate site on Highway 281 about two miles west of Los Indios, but this is almost a mile from the river and cannot be the correct location.

Since 1846 the Rio Grande River has meandered considerably, and today the ground for whose defense the U.S. officially went to war against Mexico has almost certainly reverted back to Mexico.


Lamont Wood is a freelance writer living in San Antonio, Texas, who writes about both technology and history. He has written hundreds of articles for magazines and is the author of nine books.

Originally published in the November 2014 issue of Armchair General.