They've been rebelling against the U.S. backed dictator since he replaced the last U.S. backed dictator in Yemen.
Yemen is one of those places we have been exploding ordinance for a while. For some reason it isn't a newsworthy war. I suspect there are no 'good guys' to try and even dress up.
Obviously it hasn't had the megatonage necessary.
There has been a civil war in Yemen off and on for as long as I can remember.