First of all, the U.S. did not declare war on Germany. It was the other way around. Hitler was attempting to show good faith and declare war on the U.S. in the hopes to lure Japan into war against the USSR. But in the spirit of the question, I do not believe the UK could have done any real damage to Germany. The USSR would have taken an added 4 years to the war because of the lack of a second front and lack of material support from the U.S. Would they have defeated the Third Reich.....probably not because Germany would have had the time to allow their wonder weapons, jet planes, advanced subs to have an impact against the allies. Like the saying goes, the industrial might of the U.S. is what tipped the balance.

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American by birth, TEXAN by the grace of GOD!