Sentence:  But some discussion of the complex <WSD>relationship</WSD> 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 causality. 
Meaning of the word marked with <WSD> tags:  Connection or association; the condition of being related.