As I understand it, the only way to do book a Reaction is to Pause the AI once you've completed your turn in Military phase, then select your Army & book. But, fiddling with the Tannenberg scenario for example, as Germans -- it would be useful to be able to book a Reaction to interrupt the coordination between Russian I and II Armies -- but I just can't hit Pause fast enough! They coordinate so quickly there's just almost no way to end your phase and hit Pause fast enough to interrupt !
It just seems like a clumsy mechanic, particularly when other actions seem straightforward (like booking Interceptions, which you are allowed to do during your own phase -- I just don't get why you can't do the same with booking reactions). Am I missing something?