The alliance between the two countries really only formed or solidified in the 20th century. Partly it has to do with similar forms of government and similar ideas and philosophies about government. Partly it had to do with the fact that, after the Spanish American war, the US found itself owning overseas colonies in Asia and the Pacific as well as the Carribbean, which gave some more common ground between the countries. It also made for common interests, in no small degree. However, the US had its splendid isolation from European affairs, and while the British worried about the German naval build up, the US looked to the East, and when the First World War broke, while there was a good deal of sympathy with the British (and Begians), most Americans were happy to stay out of European affairs and mind our own business. It wasn't until a combination of U-boats and dimplomatic stupidity enraged the American people against the Germans that the US went into the war, and still we saw ourselves as very much as closely allied with the French as with the British. I think that the cardinal turning point, the one that cemented the US / UK as allies be default, as it were, was the relationship between Churchill and Roosevelt and the whole Second World War. The agreement to focus on Germany, even though most Americans were most angry at the Japanese after Pearl Harbor, and the previous Lend Lease program helped to save Great Britain. The fact is that the army that fought through Europe from the Italian invasion on to the end was very much a joint Anglo-American force, from the top down. Finally, after the war was over, and the US saw that they would have to have a hand in European affairs for a long time, and that Britain, exhausted by the war and impoverished, would require US help to balance European affairs was the last brick.