Supply or not, it is still unusual for an anti-tank and a recon unit to kill a corps in 2 hits. Especially if it was entrenched. I wonder if there were some additional air attacks as well, maybe that you didn't see. I have been having trouble with the replay recently, sometimes it hiccups and skips some enemy attacks or movements(especially air attacks for some reason). It is possible that the unit was bombed and damaged before the ground attacks, rendering it even weaker than the poor supply would dictate.
You may also want to set your HQ's to "auto-assist", and make sure that units in the front line are always attached to an HQ directly, as they are the most likely to be attacked. The AI doesn't always make the greatest choices assigning units to your HQ's, make sure you are doing it yourself. This will help raise your readiness as well.
Thanks Markiss. I thought it was unusual too, which prompted me to post on the forum. I'll keep an eye on the replay, but I didn't notice any Axis bombers at all. They only had a fighter. The things is, I had 3 tac bombers and escorts/interceptors, so I honestly didn't think their air force would pose a problem.
My units were also entrenched. I tried to get the engineers to build some fortifications, but it seemed more trouble than it's worth.