There are few problems in movies that a good old-fashioned weapon can't solve. Terrorist taking over a skyscraper? John McClane solved that one with a couple guns and some pithy one liners. Aliens blowup the White House? Nothing a couple fighter jets and a computer virus can't solve. The entire Russian military after you? A tank should be able to fix the situation (well, if you're James Bond that is).
But the only problem traditional weapons can't seem to solve is Godzilla. No matter how many times he and his kaiju buddies storm Tokyo, the military always brings out the machine guns, tanks, fighter planes, and helicopters. And no matter how many times they do, they never seem to work.
To answer this, we'll need to remember that Godzilla represents the nuclear weapons. So every time the Japanese Self-Defense Forces go up against Godzilla, it's really them going up against the bomb. And if the weapons represent the Defense Force, then the result is to show the inadequacy of defense from the power of nuclear weapons.
When Godzilla assaults Tokyo, soldiers fire machine guns and tanks roll up to blast the reptilian beast back to the Stone Age—well, we suppose Godzilla would have to be blasted further back than the Stone Age since it's a dinosaur, but you get our point. But the soldiers can't scratch Godzilla, and the tanks are merely stepping stones in its path.
Later, the military lets loose the fighter jets and their missiles. The jets manage to annoy Godzilla more than the tanks and machine guns, but even they do nothing to stop the beast. In the end, despite using the full might of the Japanese Self-Defense Forces, Tokyo is left to smolder into ash.
Let's not forget a key point here: the military seen in this film is the Japanese Self-Defense Forces. It isn't designed for expansion or to invade other countries. Its purpose is simply to defend its homeland. Yet, faced with the force of the nuclear bomb, this military and its weapons can't defend the populace. All defenses are inadequate when put against that power.
We can't believe we're saying this, but even the power of John McClane and James Bond isn't up to that task. It's that terrifying.