America didn't win the battle of the Atlantic by itself, it was fighting along side British and Canadian vessels. Canada played a crucial part in that battle, without Canada the battle of the Atlantic would have been lost. You can't say that America saved Britain. Britain was saved by a collective of allied countries, not just America.
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My point being that without America joining the war Britain would have been cut off and eventually starved into submission(which was very close to happening) they would have had to surrender or would have crumbled when Germany inevitably invaded. Winston Churchill actually called Pearl Harbor as the best thing that could have happened because the sleeping American giant would have been awakened and that the allies have won the war. Not the actual quote but very close to it. If him saying that we saved Britain doesn't convict you than I don't know what would.