When war broke out in Europe, in 1914, what was the significance of U.S. policy?
The major significance of U.S. policy at the outbreak of war was that it remained nominally neutral. At such time as the U.S. did intervene in the war, it proved to be the deciding factor as it brought its vast resources and manpower to bear which no other nation could match; in fact when it was apparent that the U.S. would enter the war in the near future, Germany and the Central Powers did their best to conclude the war before this intervention took place; as they knew that if this happened, the outcome would be inevitable. The point is made obvious by the fact that the war ended shortly after the U.S. entered. Thus one may argue that the war was prolonged by U.S. refusal to intervene earlier.
The importance of U.S. neutrality is made manifest from the warnings which the German government published in U.S. newspapers before ships such as the Lusitania sailed. Similarly, following the sinking of the Lusitania, which was a British ship although with Americans aboard, the Germans issued the famous Sussex pledge whereby they promised not to sink ships without proper warning, a departure from the Unrestricted Submarine Warfare which had worked well for them previously. Both sides knew the importance of U.S. neutrality, so the Germans attempted to preserve it while the British worked to end it by cutting the cable between mainland Europe and the U.S. so that the only news of the War reaching the U.S. passed through British sources.
On the domestic front, U.S. neutrality was important in the re-election of Woodrow Wilson to the Presidency. His campaign slogan at that time was "He kept us out of the war."