On this day in 1916, the Austrian army launches a major offensive operation against their Italian enemies on the Trentino front, in northern Italy.

After considering their options carefully, and weighing offers from both sides, Italy had accepted considerable promises of post-war territory from the Allies and declared war on Austria-Hungary (but not on Germany) on May 23, 1915. This opened up a new front in World War I, stretching 600 kilometers—most of them mountainous—along Italy’s much-contested border with Austria-Hungary in the Trentino region. Upon declaring war, the relatively ill-equipped Italian army immediately advanced into the South Tyrol region and to the Isonzo River, where Austro-Hungarian troops met them with a stiff defense. The snowy and treacherous terrain made the region poorly suited for offensive operations, and after several quick Italian successes, combat settled into a stalemate.

The Austrian offensive of May 15, 1916, began with an opening bombardment of the Italian positions by nearly 400 guns. Though they resisted gamely, the Italians were driven off the mountain peaks and forced to retreat south of the town of Rovereto. Nine days after the offensive began, a heavy snow fell, putting a halt to the Austrian advance before they could capture the 4,000-foot peak of Mount Pasubio. Within a week, however, the offensive resumed, and the Austrians continued their resolute advance through the mountain peaks and passes. By the final day of May, thoroughly exhausted but triumphant, they had captured 30,000 Italian prisoners and gained a total of 12 miles of territory since the start of the offensive.