Sentence:  But some discussion of the complex relationship between “allohistory” and sf is appropriate here, as the genres overlap in certain ways. Classical allohistory— such as Trevelyan's "What if Napoleon had won the Battle of Waterloo?" and Churchill's "If Lee had not won the Battle of Gettysburg" —is a rigorously consistent thought-experiment in historical <WSD>causality</WSD>. 
Meaning of the word marked with <WSD> tags:  The relationship between something that happens or exists and the thing that causes it; the cause and consequence relationship.