I think the reason why the British and the Americans hesitated so long before reacting against Hitler lays deep into the culture of the Anglo-Saxon world. Communism was a far bigger threat than fascism/nazism could ever be.

1. For both the US and the UK Russia has always been a bigger enemy than Germany. In fact, it wasn’t untill Germany was building a fleet the UK began to change its policy of splendid isolation. In the 19th century, both the UK (Afghanistan + Dardanelles) and the US saw Russia as a bigger threat to their own security. Even after WWI, the allies were still convinced a (relativelly) strong (economically) Germany was necessairy to keep a balance in Europe. It wasn’t untill after WWII this policy has changed thanks to NATO “to keep the Americans in, the Russians out and the Germans down”.

2. The UK and the US had nothing to fear from nazism or fascism. The ideology would never reach that far, in contrary to communism. In the 1920s the communist party in the US was becoming popular. However, a nazist ideology based upon race and blood would never find any support in the melting pot the US was or even colonial Britain. The UK and the US didn’t have the same roots of romanticism nazism is based upon. But socialism (certainly at that time) could be perfectly possible in the liberal society of the US or even the UK, based upon rationalism (all men are equal …).

Actually WWII was not just fascism vs a non-fascist alliance. It was fascism vs communism vs democracy. 3 parties which were as different from one another. The finest example were the two A-bombs. Actually they were dropped upon the wrong party/country. They should have been dropped upon the USSR, to scare of the Soviets. One has to be very naive believing these two bombs were necessairy to win the war in the Pacific.
Japan was eager to surrender on just one condition: Hirohito should be kept as the head of the state. I’m not picking a side here, as the USSR would probably have done the same.

A different approach for the same answer. I don’t believe in military analysis or political science as the final answer, but more in the broad cultural history (Huizingha) which is a lot more modest. I don’t think one can answer this difficult question with just political “facts” though. There’s more than that. Even though many political scientists and historians of today claim Chamerblain was a weak figure for not pulling Britain into a war at that time, one has to realise the horrors of Passendale, Ypres … were still fresh in the memory of the British public. I think it’s a human thing to hope for the best. Unlike the beginning of WWI, when WWII broke out, everyone knew it would be even worse. WWI was started in the optimistic belief it would be over by X-mass thanks to modern technology. When WWII broke out all knew this modern technology could only mean a war even greater, more destructive than the one which was “just” over. I tend to agree with Eric Hobsbawsm’s definition of the “31-year war of Europe”, which started with a bullet (on Franz Ferdinand) and ended with another one (Hitler who killed himself). The phoney war was just wishfull thinking, hoping it would not lead to another massive war. The same would happen today I guess. To quote Einstein “I don’t know with what kind of weapons WWIII will be fought with, but I know for sure WWIV will be fought with sticks and bricks …”