What event caused napoleon iii to believe his invasion of mexico could be successful?
a. the u. s. civil war
b. the monroe doctrine
c. the congress of vienna
d. the mexican revolution

It was the U.S. Civil War.  Napoleon thought that while the U.S. was preoccupied with its own conflict no one would interfere as he made plans to invade Mexico.  Although he was able to do that, the Mexicans resisted fiercely.  Eventually the Civil was over and he was forced to withdraw.

i think the answer would be a