Thus we may have been British until the 1940's when Britain lost her other colonies.
Keep in mind that the main reason Britain released her other colonies was that the British saw the United States as an example, and realized we were much more beneficial to Britain as an ally and trading partner than as a colony which they had to protect, manage, and keep under their thumb. So without the American Revolution, it's highly possible that the British Empire would still exist--or if not, that each of these countries would have won their independence through war and gained very anti-British governments, or that some other nation would be the superpower of the world besides the US.

As to the general question, I'd say both have to be present. I believe that without a leader who was both as fanatical and as charismatic (and face it, he had the country eating out of his hand) as Hitler, the Nazi party never would have risen to power--but on the other hand, without the extreme poverty Germany suffered after WWI, he would never have risen to power. And without Neville Chamberlain continuing his appeasement policies, Germany would have still been a poor nation instead of having the wealth to carry out her dreams of conquest. And without Britain's heavy anti-war sentiment, Chamberlain would never have come to office.

Likewise, without the tyranny of the Czars, there would have been no revolt. But without Lenin and Bolsheviks destroying all opposition after the revolution, the Russian government might be something very different. Without WWII Truman wouldn't have been able to let Russia have half of Europe and wash the Hitler-Stalin pact down the memory hole, and without those nations the USSR may never have risen to the power it held. Likewise, hadn't Truman denied assistance to Chiang Kai-sheck and given loans to Mao Tse-Tung, China would not have become a communist nation, yet without America's heavy isolationist sentiment Truman may never have been elected.

It depends on the instance, but both are interdependent.