For most of the United State's life, it has attempted to go by the words in President Washington's farewell address. These words were interpreted to mean, "Do not form permanent alliances". So the US generally stayed out of European conflicts unless the conflict was brought to the western hemisphere. Continuing with this policy, when World War I began in 1914, the US tried to remain neutral, and see to it that its rights as a neutral power were not violated. Their shipping rights as a neutral company, however, were violated. The violation of American shipping rights by the change in German naval policy in 1917 in addition to America's economic interests and allied propaganda brought the US into war in 1917 against Germany.
From the start of the war, Britain's surface naval power was unquestioned by any of the Central Powers. So the Germans relied on their naval power that was not on the surface or submarine power.
The submarine, while not very effective against large surface ships, was able to devastate smaller merchant vessels headed for the British Isles. At the start of the war, German naval policy was to sink any ship coming to the British Isles that was containing weapons or other such war goods. However, as the war progressed, passenger liners also began carrying these war goods. Because German submarines were at risk when surfaced, their only option was to sink first, and ask questions later. When first confronted by American's about their unrestricted submarine warfare, the Germans toned their tactics down a little, but when the risk of losing the war became greater than the American's joining the side of the British, Germany resumed it's submarine warfare. This submarine warfare heavily turned public opinion against Germany.
Public opinion was crucial to...