It would probably be more correct to say that the British needed the U.S. to enter World War One on their side rather than the other way around. The British did all they could to ensure that if the U.S. came into the war, it would be on their side. They cut the cable allowing communication between the European mainland and the U.S. thus insuring that any information reaching the U.S. came from England. The British hyped stories of German atrocities to swing American public opinion to their side. Finally, it was the British who intercepted the famous Zimmerman Telegram and made sure that the Americans saw it. Their motives were not completely altruistic.
Even so, Americans were already inclined to support Great Britain during the war:
- British and Amerian people shared a common language and culture.
- The British were perceived as democratic lovers of freedom and the Germans as autocratic advocates of despotism.
- Most American military officers supported the British army, as they considered the German military to be a threat.
- Additionally, Woodrow Wilson, then President of the U.S. made no attempt to hide his disdain for those who immigrated from Eastern Europe. He frequently referred to them as "hyphenated Americans."
Thus it was something of a foregone conclusion that if the U.S. entered the war, it would be on the allied side. All the British needed to do is nudge the U.S. a bit further along.