I suppose this is a bit philosophical, but I'm writing an essay that involves U.S. intervention/non-intervention in Libya and Syria respectively in recent years. It strikes me that the relative success/failure of those rebellions is incredibly contingent.
How are we to regard close-run events with profound outcomes that might have very easily turned out differently?
Some quick thoughts, not a comprehensive answer:
I suggest starting with Andrews and Burke's 2007 article "What Does It Mean to Think Historically?", which begins to address how we deal with the "five C's" of historiography: change over time, causality, context, complexity, and contingency.
It's always hard, and while historians like to dismiss counterfactual history as mere fantasy, the fact is that every statement of historical causality (ie "The Union won because of X, Y and Z"), contains an implicity counterfactual statement.
Two books that I read which I found shaped by thinking on the problems of contingnecy and causality were David Hackett Fischer's Historians' Fallacies: Toward a Logic of Historical Thought and Sam Wineburg's Historical Thinking and Other Unnatural Acts: Charting the Future of Teaching the Past.
Your questions is a bit philosophical, but it also gets at the heart of problems that historiographical scholarship has been struggling with since the 1970s.