Which is exactly why the "military base overrun by zombies" scenario is bunk, bull****, inconceivable, impossible, <pick an adjective>, etc.
They don't operate without support. There are more tanks. There are helicopters. There are artillery batteries. There are regiments of infantry support.
there's not a doubt in my mind that a military base could wall themselves off from the zombie hordes. but at some point they will need to leave that base. they would need to go on recon, hunt down survivors, find supplies, etc. they would no doubt use their tanks, apcs, helicopters up to the point that fuel becomes an issue. when it does then they would have to send soldiers out on foot.
this opens up the possibility that some of those soldiers would be wounded during the course of their mission. it is likely that those wounded soldiers would be brought back into the base, where they may become zombies. if the military are aware of what it means to get bitten, they would likely quarantine anyone who was wounded, concentration camp style. then enter human nature. in zombie movies people are always concealing that they have been bitten, people keep wounded family members around hoping they might recover, possibly unknowing to anyone else behind their perimeter. you would not only need to control the perimeter, you would also have to make sure nobody is doing anything stupid.
i think i would prefer mobility over fortification. id take a light armored vehicle over a tank. a humvee with a machine gun turret would be my perfected vehicle. id keep a load of mres, water, fuel, weapons and ammo. with doors and windows (4 inch thick windows) locked, there would be no way a small number of zombies could get in. a large number may be able to flip the vehicle. id use it to try to find a ship, which i would park some distance off the coast and use as a base. even if the zombies could swim, it is unlikely they could climb onboard. i would use that as a base of operations, if the area around it gets overwhelmed by zombies, it can always be moved.