Union couldn't attack Gundam because the only way is to get visual on Gundams (NO radar, heat(no exhaust) or sound(no engines) and maybe infared going to work to detect them). So how are you suppose to defend when you are ordered to attack country and you have no idea from where Gundams will come and who they will attack? And when you figure out who they are attacking, you are died. Taribia's forces were counting on Gundams for them to fight. Without Gundams they are just waiting for bombardment from ships to be recycled to local background.