The event that helped draw USA into First World War: - the sinking of the British luxury liner Lusitania
Back in 1915, the war was raging in Europe as Britain and its allies fought Germany and its allies. The United States was officially neutral. The Lusitania's 1,257 passengers and 667 crew members were in high spirits as they set sail from New York City on May 2, bound for Liverpool, England. Their steamship was the world's largest and fastest liner, known as the Empress of the Sea.
The day before, on May 1, Germany had placed a notice in U.S. newspapers warning that the war zone included the waters around the British Isles. Nevertheless, it seemed that the Lusitania's trip would be a smooth and sunny one. On May 7, as the passengers finished lunch, they glimpsed the green fields and white cottages of southern Ireland.
Suddenly, a torpedo from a German submarine ripped through the side of the Lusitania, and the ship began to capsize. Within twenty minutes the Empress of the Sea had sunk. Among the 1,198 who died were 124 Americans. The unprovoked attack was widely condemned. Germany claimed that the Lusitania was armed and carrying ammunition to the British. Britain said the Lusitania was unarmed and carried only a small amount of rifle ammunition.
Support for Britain increased after the attack, and "Remember the Lusitania" became a common slogan. Two years later, on April 6.1917, the United States declared war on Germany.(Grolier Book of Knowledge Encyclopedia)