Reason vs. Rationale?

I have to write a paper for my History cl and part of the prompt says: "In each of these three wars, you should examine what interests of the U.S. were at stake and consider whether the reasons for intervention were the same as the rationale for intervention? (Be sure you know the difference between a reason and a rationale)." I always thought "reason" and "rationale" were synonyms and I haven't been able to find a clear difference between the two just by looking up the definitions. Can somebody please explain what the difference between reason and rationale is?