On February 3, 1917, President Woodrow Wilson formally cut diplomatic ties with Germany. This happened in immediate response to the German declaration that they would resume unrestricted submarine warfare.
Since the start of World War I, the United States had done its best to maintain neutrality. However, the German navy's practice of sinking ships, including some American ships, without warning made neutrality difficult. Several times, including after the sinking of the Lusitania which killed 128 Americans, the United States severely warned the Germans that they would not stand for such acts. For some time, the German navy restricted its use of submarines in response.
However, in early 1917, German strategists came to believe that they could quickly force Britain to surrender by resuming unrestricted submarine attacks in British waters. German leaders decided to take the gamble that they could win the war before the United States had time to get involved. On January 31, 1917, German Chancellor Theobald von Bethmann Hollweg announced that submarine attacks would resume. This led to Wilson's response three days later.
Wilson made it clear that he had no desire to go to war against Germany. However, if Germany would continue sinking American ships and putting the lives of Americans at risk, he would have no choice but to ask Congress to declare war. After Wilson made these remarks to Congress, Count Johann Heinrich von Bernstorff, the German ambassador to the United States, was ordered out of Washington. Shortly thereafter, James Gerard, the US ambassador to Germany, was called back to the US.