That’s a seriously academic question, meaning who the hell would ever ask that except a history professor, and even then, for what reason? And, could there even possibly be an actual answer? Is the teacher trying to make you all hate history?
It’s just an excuse the give you something to write about. The most obvious external forces are the assassination and the treaty, diplomatic and military actions of the wars the assassination sparked. Before the war and during it there were of course also internal forces (though of course that’s an invented concept too), such as economics, politics, and cultural traditions.
It doesn’t matter which had more influence, though, even for the assignment. The question just provides an opportunity for you to show you know things about the period, and that you can form arguments that use that knowledge. Go find all the events you know about that involve internal or external “forces”, find more if you don’t have enough, then pick the most interesting ones, and write about them, comparing them to each other and weighing them or whatever makes sense to you, followed by some sort of conclusion.