Why did the Iroquois ally themselves with the British?

I don't think this is a valid question. Britain was at war with Germany from September 1939, the US only after Pearl Harbour. So the US did not "choose" Britain as an ally, we simply ended up with common enemies, ie Germany and Japan.

Between September 1939 and when the US entered the war, Britain had very little help from the US, which stayed neutral in this period, and would probably have stayed that way if Japan had not attacked. In that case it would have been doubtful if Britain could have defeated Germany. Apart from being 2 countries separated by a common language..... The US did help Britain considerably before Pearl Harbor, Lend-Lease was crucial. The policy of defeat Germany first was also necessary, not just for Britains aid, but that of Russia and the whole of occupied Europe. Although the effort put into D Day by the British was immense, I agree with the above, the British couldn't have done that alone. Europe would be a very different place without the US joining the war.