Defense is just a polite term for war.

What should the ratio be of missiles to missile interceptors?

There is this mistaken idea out there that if the U.S. were to disarm, then other nations would also disarm, and the whole world would live in peace. But history tells us over and over that power abhors a vacuum, and were the U.S. to leave the world, another would take its place, probably Russia or China. If we disarmed, then another nation with arms will inevitably invade the U.S.