My old hiistory teacher said that the hows, whys and whens were all good and well but the most interesting question in history is 'what if'. Energised all of us (17 yr olds) and has stayed with me ever since. Shame more people drop out of history at 16 (or, in spirit, earlier) due to a lack of a similar moment.
"But history is contingent, and the only way to fully appreciate the significance of a given event is to think about what might have happened if things had turned out differently" is exactly right. Being able to talk about a what if and base it in evidence is real understanding.
Far too many people, often encouraged by political narratives, think of history as a deterministic procession to the 'inevitable' present day, and have no concept of how freak chance and human error/inspiration play havoc with the best laid schemes of mice and men.