The enemy didn't necessarily break the Geneva convention.
Pulling the trigger far in advance of the resultant action increases the risk of disaster, disaster that could've been averted based on the richer dataset available closer to the scheduled time.
They didn't have to. We're just going to say they did anyway because they are evil bastards (TM) and we can't possibly be anything but the good guys.
This scenario is the exact same scenario as a current ballistic missile launch. There are no safeguards for those systems that could be intercepted and interfere with the use of the weapon.
Pulling the trigger far in advance of the resultant action increases the risk of disaster, disaster that could've been averted based on the richer dataset available closer to the scheduled time.