The main problem with the reinforcements mechanic is that of the “pinning” trick, whereby the attacker can send a single infantry to attack stack B in order to prevent units defending stack B from reinforcing stack A during that combat phase.
Possible solutions:
1. The defender always chooses the order in which battles are resolved;
2. An attack is not allowed against a force more than 3 times that of the attacker (in total units);
3. The defender is still allowed to reinforce even from tts under attack; this creates other problems, effectively giving the defender a “free retreat” option.
4. Fight all battles in a combat phase simultaneously by round, only allowing reinforcements from tts clear of enemy units.