How much of history is decided by so-called "great people" and how much is just the situation that creates them?

For a few examples, if Hitler never existed, would there be another right-wing extremist to take his place or could only Hitler fill that role? If Napoleon hadn't seized control of France, would another military commander have gained power in an unstable France or could only Napoleon have done so? If George Washington hadn't existed to lead the Continental Army, would another general have risen or could only Washington be the leader? If Caesar hadn't waged war against the Roman Republic, would another general have taken his place or could only Caesar seize power?

Basically I'm asking, for a given political situation, how much influence do the major powers involved actually have over the outcome? does a political situation demand certain men to rise into power or do the men themselves create the situation to gain power?

What you are asking about is called Great Man Theory and while certainly there will be preferences in academia overall for or against certain theories the truth is that it is opinion (albeit based on evidence and logic).

None can answer "what ifs" as a fact by the nature of the question. What I will say (with my very limited "knowledge" in this area) is that this question could be argued as philosophically as much as historically.

The main idea's are

1. An individual shapes a time to their will (great man)
1. The times and general opinion of a populace creates a void which people will naturally fill (i.e. WWII would happen in some format with or without Hitler due to the issues of the time).
1. A mixture of the two favoring one side or the other with luck.

I must again state though that I believe this question can not be answered in an absolute manner only discussed. At best what I wrote above is an overview which others more suited can add to or indeed correct as they see fit.