Yankton soldiers ready to fight in World War I.

America could be considered something of a reluctant superpower. But circumstance brought it to the world stage -- essentially against its will.

When war broke out in Europe in 1914, America vowed to remain neutral in part because of severe ethnic splits in loyalties at home.

But the sinking of the British liner Lusitania in May 1915 by Germany signaled a shift in U.S. sentiments, as 114 Americans died in the disaster. Germany refused to disavow the attack. The Germans' continued U-boat activity drew increasingly stronger protests from the United States, which eventually broke diplomatic relations with Berlin.

In February 1917, a correspondence known as the Zimmerman Telegram was intercepted by the British Secret Service. In the telegram, German Foreign Minister Zimmerman secretly invited Mexico to join in the war on the side of the Central Powers (Germany, Italy, Austria-Hungary) and against the United States, which now was leaning toward the siding with the alliance of Great Britain, France and Russia. In return, Mexico would receive the lands of New Mexico, Arizona and Texas. The British, fearing the note might be dismissed as a ruse to draw America into the war, presented U.S. President Woodrow Wilson the note only after the United States broke off diplomatic relations with Germany.

In March 1917, five U.S. ships were sunk by German U-boats without warning. That prompted Wilson to ask Congress to declare war April 2. "The world must be made safe for democracy," he stated.

Ultimately, more than 116,000 Americans died during the war, either in battle or from other causes related to the war.

Thus began America's official participation as a combatant in the Great War. It was called the War to End All Wars, but subsequent events years later would reduce this conflict's title to a number.