1. World War I

"The Great War was without precedent ... never had so many nations taken up arms at a single time. Never had the battlefield been so vast… never had the fighting been so gruesome..."*

In 1917 when the United States of America finally entered World War I - the Great War, as contemporaries called it - Europe had been been ablaze since August of 1914. It seemed that as soon as the smoke cleared from a revolver used by a Serbian nationalist to assassinate Archduke Franz Ferdinand (heir apparent to the Austro-Hungarian throne), the military alliances among Europe's powers quickly drew up sides and plunged the rest of the world into the first man-made catastrophe of the 20th century. After all, World War I marked the first use of chemical weapons, the first mass bombardment of civilians from the sky, and the century's first genocide.

The allies -- chiefly Russia, France and Britain -- were pitted against the Central Powers -- primarily Germany, Austria-Hungary and Turkey. Eventually, the War spread beyond Europe as the warring continent turned to its colonies and friends for help. This included the United States, which joined the War in 1917 when President Woodrow Wilson called on Americans to "make the world safe for democracy."