It was, after an attempt to run a smaller airline on their own. The US partnership was a last gasp reincarnation after 9/11 ATSB money (or some other money related to that) came through that didn't last very long. I had a ticket on them (that I abandoned and conceded the money) for a flight about a week before the final Midway shutdown.

http://en.wikipedia.org/wiki/Midway_Airlines
http://www.usairways.com/midway/