|The title says it all:
Who do you think has been the greatest enemy ever to threaten America?
My vote goes to the British hands down. No other country ever came as close as the British to physically ending the United States in our history.
The Germans and the Japanese were formidable in their own right, but neither [or even both] could reasonably invade the United States.
The Soviet Union had the theoretical potential to destroy the United States, but I think everyone agrees that this was not a practical capability in the way the British Empire's ability to take Washington DC was. The Soviets were a dangerous enemy ideologically in the way it could convert adherents in America, but they never out-did the British who successfully supported a rebellion in the United States by funding, arming, and giving moral support to the Confederacy.
So what country would you choose?