Great Britain and her Empire stood alone after the fall of France. At that time there was no guarantee of any help, or assistance from anyone else. There was absolutley no certainty that the US would enter the war, or indeed on which side the US would enter. There was no intimation apart from what was written in "Mein Kampf" that Hitler would attack the USSR. Great Britain could have come to terms with Hitler and Nazi Germany, very easily - that was the easy way out - Fact was they did not even consider it, instead they elected to fight and Churchill never doubted the outcome for a second.