I had an instance where two lords were fighting, and I jumped in to help my guys. We eventually won, but I was one of the few left standing at the end.
When it went back to the srategic map, however, the two armies were still fighting, even though one side had 0 available troops. I was able to join the "battle" about 5 times, each time the tactics script would freak out on me, but eventually say I won. Only after a few more tries of this did the defeated lord's army actually disappear.