On June 4, 1916, the Battle of Lutsk marks the beginning of the Brusilov Offensive, the largest and most successful Allied offensive of World War I.

When the fortress city of Verdun, France, came under siege by the Germans in February 1916, the French pleaded with the other Allies, Britain and Russia, to mount offensives in other areas to force the diversion of German resources and attention from the struggle at Verdun. While the British plotted the offensive they would launch near the Somme River in early July, the first Russian response came more quickly—a failed offensive in March at Lake Narocz, in which Russian troops were slaughtered en masse by the Germans with no significant effect at Verdun. Still, the Russians plotted another diversionary attack in the northern region of the Eastern Front, near Vilna (now in Poland).

While the Vilna offensive was being planned, General Alexei Brusilov—a 63-year-old former cavalryman and aristocrat given command of the Southwestern Army (the Russians divided their army into three major groups, Northern, Eastern and Southwestern) in March 1916—pressed his superiors at a meeting in April that he be allowed to attack as well, although no action was planned for the southwestern section of the front. At the very least, Brusilov reasoned, his attacks would draw troops away from the other area and ensure the success of their offensive in the north. Though he was given the go-ahead, the other Russian generals had little confidence in Brusilov’s strategy.

Brusilov’s troops began their attacks on the Austro-Hungarian 4th Army at the city of Lutsk (now in Ukraine), on June 4, 1916, with an impressive bombardment from nearly 2,000 guns along a 200-mile-long front stretching from the Pripet marshes to the Bukovina region to the southwest, in the foothills of the Carpathian Mountains. Though the Austrian troops at Lutsk, led by the over-confident Archduke Joseph Ferdinand, outnumbered the Russians—200,000 men against 150,000—the success of the barrage obliterated this advantage, along with the Austrian front line, as Brusilov’s troops swept forward, taking 26,000 prisoners in one day.

Within two days, the Russians had broken the 4th Army, advancing 75 kilometers along a 20-kilometer-long front, and effectively ending Joseph Ferdinand’s career. Some 130,000 casualties—plus the capture of over 200,000 prisoners—forced the Austrian commander, Conrad von Hötzendorf, to close down an offensive against Italy in the Trentino region to divert guns and divisions back east. On June 15, Conrad told his German counterpart, Erich von Falkenhayn, that they were facing the greatest crisis of the war so far—a fact that took Falkenhayn, who was optimistic about an imminent French surrender at Verdun, completely by surprise. Confronted with the Austrian panic against Russia, he was forced to release four German divisions from the west, a weakness that allowed a successful French counterattack at Verdun on June 23, just one day before the preliminary British artillery bombardment began at the Somme.

Dubbed The Iron General and respected and beloved by his troops, Brusilov relied on absolute preparedness for battle and on the execution of even the most minute detail of his orders. The June 4 attacks began a string of crushing victories against the Austrian army across the southwestern portion of the Eastern Front, forcing Germany to abandon plans for their own 1916 offensive in France in order to bail out their hapless ally—even as they confronted a new British offensive at the Somme in July. By September, Russian resources had began to run out, however, and the Brusilov Offensive reached its limits; it was shut down on September 20, 1916, having cost the Austro-Hungarian army a staggering total of 1.5 million men (including 400,000 taken prisoner) and some 25,000 square kilometers of territory.

Though turmoil and revolution shattered Russia in 1917, disintegrating its army and leading to its subsequent exit from the war—a fact that caused the success of the Brusilov Offensive to be largely forgotten—the offensive permanently secured more enemy territory than any other Allied offensive on either front. Moreover, a permanently debilitated Austria-Hungary never again played a significant role in the war. Its army was reduced to holding trenches against the weaker Italians, and Germany was left to fight virtually alone for the final two years of World War I.