From the very start of the conflict, the United States attempted to remain more or less neutral in World War I. Most Americans saw the war as a European matter that did not concern them. President Woodrow Wilson agreed with this opinion and officially declared the United States to be neutral in the matter. He was particularly concerned that the diverse ethnic backgrounds of Americans could lead to confused senses of loyalty if the country were to enter the war.
However, remaining neutral was more easily said than done. When Germany began its blockade of Great Britain, trade between the United States and Britain was greatly threatened. The United States and Great Britain had long been major trade partners. The United States hoped that this relationship could continue despite the war.
In February 1915, Germany announced that it would attack any ship in British waters, regardless of nationality. It was not long before several American merchant ships were damaged or sunk. Wilson condemned these attacks, but the Germans did little to appease the Americans. When a German submarine sank the passenger ship, the Lusitania, 128 Americans were killed. Despite German apologies, unrestricted submarine warfare continued.
As all this was happening, it became apparent to many Americans that it would be impossible to sit out the war. However, the US military was in no condition to enter a conflict of this size. Politicians, military commanders, and policymakers got to work getting the country ready to fight if need be. The National Defense Act of 1916 authorized the expansion of the US military. The president was given the authority to federalize the national guard and the army began developing an aviation program. Government propaganda urged many young American men to enlist. However, few did.
Many in military leaders and members of Congress were afraid that the country would be caught flat-footed should their hand be forced. That moment came in January 1917 when the United States was made aware that Germany was encouraging Mexico to declare war on the United States. This, along with the continued risk to American ships from German submarines, led Wilson to make an abrupt departure from his earlier non-interventionist stance. War was declared in April, and Wilson called on Congress to institute a draft. As a result, the Selective Service Act was enacted in May 1917. Over two million American servicemen were sent to Europe to fight against Germany and its allies.