On this day 100 years ago, President Woodrow Wilson declared war on Germany, thereby entering the U.S in World War I.

The sudden entry into the politically isolated U.S was unexpected. In fact, when President Woodrow Wilson ran for reelection the previous summer, he had largely campaigned on an anti-war platform (his reelection campaign slogan was “he kept up out of war”). Two and a half years into World War I, however, the president was finding it more difficult to remain neutral.

A month prior, the British intercepted the Zimmerman telegraph, which was a letter from Germany to Mexico instructing the U.S southern neighbor to begin battle on the American boarder should war erupt (for more background on the coded telegram, see our story here).

After the Senate voted 82 to 6 to declare war, the House of Representatives voted 373 to 50, thus formally entering World War I.

With the entry of the United States in the European conflict, the war was quickly brought to a close, but not before it would become one of the most deadliest wars in United States history.