Two separate events on September 5 laid the groundwork for America’s eventual entry into World War I. In Washington, D.C., President Woodrow Wilson ordered that the U.S. Navy provide wireless stations in Europe for direct transatlantic communications. Wilson allowed belligerent nations on both sides to use the wireless to send encoded messages, a gesture of American neutrality that Germany later abused by using the line to propose a military alliance with Mexico against the United States. Revelation of this, the “Zimmerman Telegram,” would be decisive in Wilson’s decision to declare war on Germany.

In France the German offensive was stopped cold at the First Battle of the Marne. French troops who had been driven mercilessly backward throughout the month of August rose up and destroyed Germany’s plan to overrun France and end the war quickly. “French élan, just when it is on the point of being extinguished, flames up powerfully,” wrote the German commander Helmuth von Moltke during the battle. The kaiser’s armies were forced to retreat to the Aisne River and dig the trenches that would characterize the next three deadlocked years on the western front. This failure compelled Germany to rely increasingly on submarine warfare in the Atlantic, a strategy that backfired in 1917 when their U-boats sank several unarmed American ships and convinced Wilson that war was unavoidable.