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On April 2, 1917, President Wilson asked Congress to declare war against Germany and the Central Powers. The United States would cast its lot with the Allies four days later. What caused President Wilson to abandon his policy of neutrality?

Germany’s policy of unchecked submarine aggression against shipping interests headed to Great Britain helped bring the United States into World War I. After the sinking of the British ocean liner Lusitania by a U-boat in May 1915, widespread protests helped turn the tide of American public opinion against Germany. On May 6, 1916, the German government signed the so-called Sussex Pledge, promising to stop the indiscriminate sinking of non-military ships. Less than a year later, however, they announced the resumption of unrestricted submarine warfare, resulting in Wilson breaking diplomatic relations with the German government. In February 1917, Congress passed a $250 million arms appropriations bill intended to make the United States ready for war. The following month, Germany sunk four more U.S. merchant ships.

The march to war was also accelerated by a notorious letter penned by German foreign secretary Arthur Zimmermann. On January 16, 1917, British code breakers intercepted an encrypted message from Zimmermann intended for Heinrich von Eckardt, the German ambassador to Mexico. The missive gave the ambassador a now-famous set of instructions: if the neutral United States entered the war on the side of the Allies, Von Eckardt was to approach Mexico’s president with an offer to forge a secret wartime alliance. The Germans would provide military and financial support for a Mexican attack on the United States, and, in exchange, Mexico would be free to annex “lost territory in Texas, New Mexico and Arizona.” In addition, Von Eckardt was told to use the Mexicans as a go-between to entice the Japanese Empire to join the German cause. Handed over to the United States in late February 1917, the scandalous contents of the telegram were splashed on the front pages of newspapers nationwide. On April 6, 1917, the United States entered World War I.

Get a firsthand look at how a new breed of weapons like WMDs, submarines, armored tanks and air attacks changed modern warfare forever in WWI: The First Modern War. Here’s a look at some of the episodes:

  • An assassin’s bullet sparked a global conflict that quickly evolved into the deadliest war humanity has ever seen. In the chaos, a new generation of soldiers—and world leaders—emerged. Watch it all unfold in Trial by Fire.
  • By December 1914, all thoughts of a quick victory had faded. But on Christmas Eve, an astonishing event took place: Up and down the Western Front, Allied and German soldiers met peacefully in No Man’s Land for The Christmas Truce.
  • In Mystery U-Boat of World War I, see how technological achievements that streamlined 19th-century production, improved transportation and expanded science were used to efficiently decimate a generation of soldiers in the early 20th century.

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