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World War 1 was shocking in its cost of human life. Below are some of the costliest, in terms of casualties (killed, wounded, missing or captured) operations of the war. Numbers are rounded; sources disagree on exact totals.

First Battle of the Marne, Sept 6–10, 1914
Germany’s Schlieffen Plan to bypass French border fortifications by passing through Belgium and Luxembourg and swiftly defeating the French Army worked well initially. German soldiers were within 50 miles (80 km) of Paris. Their commander in chief, General Helmuth von Moltke, then changed the Schlieffen Plan; instead of going west of Paris to encircle the French capital, he sent his forces east to meet what he thought was a nearly defeated army head-on., giving the French and a British Expeditionary Force (Britain entered the war after Germany invaded Belgium.) to strike the Germans’ western flank in the Valley of the Marne. Both sides suffered heavy casualties and began digging the trenches that would define warfare in the Great War.
Allied casualties: 263,000
German casualties: Estimated to be about the same as Allied

Battles of Tannenberg and Masurian Lakes, Aug–Sept, 1914
In August, a German army encircled Russia’s Second Army near Tannenburg, East Prussia. The maneuever was so well planned and executed that only about 10,000 of Second Army’s 150,000 men escaped and som e500 Russian artillery pieces captured; the Russian commander, Gen. Alexander Samsonov committed suicide. The following month, German forces enveloped Russian general Pavel Rennenkampf’s First Army in the Masurian Lakes area near East Prussia’s border with Russia and dealt the czar’s troops another staggering defeat. The architects of the German plans, Gen. Paul von Hindenburg and his chief of staff Erich Ludendorff, were hailed as German heroes. After Russia was knocked out of the war in 1917, they were brought to the Western Front to take command there and were given extraordinary control over nearly all aspects of German civilian life.
Russian Casualties: 122,000, some 92,000 of which were taken prisoner
German Casualties: 13,000
Masurian Lakes
Russian Casualties: 125,000
German Casualties: 40,000

Gallipoli, Feb 1915–Jan 1916
Britain’s First Lord of the Admiralty, Winston Churchill, developed a plan to attack the Central Powers through their “soft underbelly” in Turkey and reopen the Dardanelles Straight between the Black and Aegean seas to Russian ships. The Gallipoli Peninsula jutted like the toe of a shoe between the Dardanelles to its south and east and the Aegean Sea to the west. On April ? British, Australian and New Zealand troops landed, mostly on the Aegean coast. The landings were largely unopposed, but British officers didn’t move their troops from the exposed beach to the heights above upon landing. By the next day Turkish soldiers and their German advisors poured down a murderous fire on the invaders, who would never penetrate inland during the campaign.
British & Commonwealth casualties: 214,000
Turkish casualties: 300,000

Battle of Verdun, Feb–Dec 1916
German offensive against French fortifications anchored at the town of Verdun and stretching to the Swiss border. Three forts were captured, but not the fortifications at Verdun, where the French commander, General Henri Philippe Petain, had declared, “They shall not pass!” The Germans opened the offensive by firing 2 million shells against a front of just eight miles. A British attack on the Somme Front and a Russian offensive on the Eastern Front forced the Germans to pull troops away from Verdun, and the final German offensive there took place July 21. During autumn, having gained artillery superiority, the French began counterattacks. By December 15 they had regained all the ground previously lost, leaving the lines as they had been before the German attacks began.
Allied casualties: 400,000
German casualties: 340,000

Battle of the Somme, Jul–Nov 1916
British offensive to break through German lines near the Somme River in northeastern France and relieve pressure on Verdun to the south, it was intended to be “the big push” that would end the war. A week of bombardments sent 1.6 million shells screaming into the German lines, but their damage was insignificant. When British troops attacked on July 1, they suffered the greatest single-day loss in all of Britain’s history: 60,000 casualties, one-third of which were killed. As the offensive dragged on, French troops came to reinforce the British. When the battles ended in mid-November, the Allies had won just five miles (eight km) of ground, but attrition was high among the German defenders, including a large number of junior officers and NCOs, which would affect their army’s effectiveness during the remaining years of the war.
Allied casualties: 615,000, approximately two-thirds of them British
German casualties: 650,000

Third Battle of Ypres (Passchendaele), Jul–Nov 1917
The area around Ypres, Belgium, had already witnessed two bloody struggles during the war, one of which saw the Germans’ first use of poison gas. Before launching the July 1917 offensive, British guns hurled 4.5 million artillery shells against the Germans’ position, but fortifications protected the defenders effectively. The first waves of attacking British were mowed down. The months of August and October saw some of the heaviest rains in 30 years, which turned the battlefield into a quagmire. Attacking soldiers at times found themselves in knee-deep mud as enemy machine-gun bullets whistled toward them; some men and animals literally drowned in the mud of the fields that had been churned up by the extensive artillery barrage and soaked with rain. The offensive gained just five miles (eight km), including the village of Passchendaele.
Allied casualties: 325,000
German casualties: 260,000

Spring Offensive (Ludendorff Offensive, Kaiser’s Battle), Mar–Apr 1918
German offensive on the Western Front intended to win the war before the American troops that had begun arriving in France could fully deploy. Planned by General Paul von Hindenburg’s Chief of Staff Erich Ludendorff, the main offensive was to be against the British forces on the Somme Front, accompanied by three diversionary attacks. Initially, the offensive sent the Allied troops reeling back, but the Germans advanced so quickly their supplies could not keep up in the muddy terrain pockmarked with shell craters. The attack fizzled with the Germans in a weak defensive position, but they had inflicted severe losses that were only made up by the arrival of American troops.
Allied casualties: 850,000
German casualties, 650,000

Hundred Days Offensive Jul–Nov 1918
Allied (American, British, French) offensive on the Western Front against German Second Army. Includes the battles of Amiens, Second Somme, Second Noyons, Second Arras, Meuse-Argonne Offensive. Pushed Germans back to the Hindenburg Line of fortifications protecting Germany. The offensive continued, broke through the Hindenburg Line, and resulted in Germany’s surrender, ending the war.
Allied casualties: 1,070,000
German casualties: 786,000