At the time, World War I was the deadliest conflict in world history, and President Wilson was determined to keep the U.S. out of it. America had enjoyed a mostly isolationist foreign policy stand, staying out of European strife. The U.S. was able to remain neutral until 1917, when it entered the war alongside the Allied powers (the UK, France, and Russia).
There were a number of factors that led to America's eventual involvement in WWI. Beginning in 1914, Germany launched a campaign of submarine warfare against Allied trade routes. In 1915, a German U-boat sunk the RMS Lusitania, a British ocean liner that had 123 Americans on board. After the sinking, Wilson issued a warning to Germany; the U.S. would not tolerate violations of international law and unauthorized submarine warfare.
By 1917, Germany's U-boat attacks were continuously escalating. Wilson then obtained a copy of the infamous Zimmerman Telegram (also called the Zimmerman Note). Germany had sent a message to Mexico urging it to turn against the U.S. and, after a German victory, Mexico would be awarded the territories of Arizona, New Mexico, and Texas. With the continuous German attacks on U.S. merchant ships and the publishing of the Zimmerman Telegram, Congress officially declared war on April 6, 1917.