IIRC, last time martial law was declared in the U.S. was in Hawaii during the entirety of WWII.
Nope. Happened in LA during the Rodney King riots, for example. Sounds like there's a serious need for it in NO, too -- looting and serious crisis management needs.
I believe it happened during the LA riots in the 60's, and may have happened in other places in the 60s too. Also belief that it was declared during the Rodney King rioting as well.