At the beginning of the 20th century the United States was the most powerful country in the world. The world leader in coal and steel production, the USA was also a major producer of raw materials. The most important of these being wheat, cotton and oil, which accounted for more than a third of all the USA's exports. With a population of over 100,000,000, the USA had the potential to decide the outcome of the First World War. However, in 1914, the country had no overseas alliances and on 19th August, President Woodrow Wilson declared a policy of strict neutrality.
Although the USA had strong ties with Britain, Wilson was concerned about the large number of people in the country who had been born in Germany and Austria. Other influential political leaders argued strongly in favour of the USA maintaining its isolationist policy. This included the pacifist pressure group, the American Union Against Militarism.
Some people in the argued that the USA should expand the size of its armed forces in case of war. General Leonard Wood, the former US Army Chief of Staff, formed the National Security League in December, 1914. Wood and his organisation called for universal military training and the introduction of conscription as a means of increasing the size of the US Army.
The war helped the USA economy with exported goods to Allied countries increasing from $825 in 1914 to $3.2 billion in 1916. This made it possible for Britain and France to keep fighting the war against the Central Powers and this influenced Germany's decision to announce its unrestricted submarine warfare policy. Opinion against Germany hardened after the sinking of the Lusitania. William Jennings Bryan, the pacifist Secretary of State, resigned and was replaced by the pro-Allied Robert Lansing.
After the sinking of the Lusitania, Wilson announced an increase in the size of the US armed forces. However, in the 1916 Presidential election campaign, Woodrow Wilson stressed his policy of neutrality and his team used the slogan: "He kept us out of the war".
On 31st January, 1917, Germany announced a new submarine offensive. Wilson responded by breaking off diplomatic relations with Germany. The publication of the Zimmerman Telegram, that suggested that Germany was willing to help Mexico regain territory in Texas and Arizona, intensified popular opinion against the Central Powers.
On 2nd April, Woodrow Wilson asked for permission to go to war. This was approved in the Senate on 4th April by 82 votes to 6, and two days later, in the House of Representatives, by 373 to 50. Still avoiding alliances, war was declared against the German government (rather than its subjects). War against Austria-Hungary was not declared until 7th December, 1917.