What if, hypothetically speaking, the Germans were able to assassinate Stalin and Churchill? Lets say Churchill right before he became elected PM, and Stalin (though that would be hard, as paranoid as he was) in early 1940, when they were still "friends". Do you think that would have changed much? A different British leader might have been less likely to keep up the fighting spirit. Chamberlain at least did not have that kind of charisma and intuition about what the people needed to hear by far.
A different Soviet leader might have hesitated a bit more before sending his troops into the meat grinder by the millions like Stalin did. Few other potential Soviet leaders would consider the huge losses Russia endured as "acceptable", and at the same known how to/ managed to rally that much support for the war. It was not simply "fight or be killed", by by appealing to religion and other pre-Soviet era symbols, a lot of Russians actually volunteered to fight.
What do you think?