Thats the problem and mind set right now, nobody plans for anything at the base you just took unless its "everyone rearm and up again!". Part of the plan for TAKING the base should also encompass HOLDING the base.
So instead of just getting the capture at the last second with the last M3, you have to have whirbs inbound for ground to air cover, a panzer or two covering the town, and a few fighters to cover the field. Make them defend and hold it 20 minutes, or until 3-4 sets of supplies from another base resupply the guns.