Have done this 3 times in the last 4 river battles. While I have a serious problem with the way reinforcements are handled now, not even my disagreement can be blamed for this issue.

Whenever an AI unit arrives, it sets it site on a "regrouping" position - which often times is on the other side of the river. Most units will plod along right in front of my defenders - taking shot and never even responding. That is bad enough - but the last few battles, it seems that one unit gets put on the map RIGHT at the edge of the water - and when it tries to move to the bridge to cross over - it gets stuck on a outcrop of riverbank. By itself - this would not be an issue - except the Unit also is in the EDGE tile - meaning it is not yet a legal target for melee or range attack units. So you end up with a single AI unit - stuck on the map where it can't move, and where you can't attack it - even with Force Fire.

The only option you have is to hit ESC and take the loss - no matter the real outcome.
This needs to be addressed as soon as possible.

And yes - the reinforcement issue is a major problem - I ranted at length on in the "rate the game" here thread.

"A mind is a terrible thing to taste!"