Thanks. I found it shortly after I posted. IMO it's not a very useful undo. I'm certain I'll get better at avoiding mistakes as I play more, but I find the movement and combat system very unforgiving. If you sequence your moves and attacks in the wrong order, one misclick can block an entire offensive. Also, nothing more annoying than accidentally attacking when the predicted results are 4-0. The undo should allow you to go back several moves. If someone choses to cheat with that, they shouldn't be playing grand strategy games. Go play Counterstrike and enable god mode...
Also it appears that if you have an unused move or attack but click on any other unit, that unit becomes spent.