This tends to be true for battles with multiple hundreds of soldiers on each side, even if reinforcements aren't required.
You can (and should) rally your troops back together in their formations by using F1 and setting the point for each unit type to regroup at. Then you can proceed to direct your well oiled military machine to grind up the random rabble of the enemy more efficiently.
Discussion on the subject of reinforcements kind of moved from this thread a few weeks go to another thread:
Then that one ran out of steam and discussion came back here when this thread got necro'd. Not that the discussion flowed or shared the same main posters, just two different threads spawned over the same issue.