Pretty much self-explanatory, you can make a Missile that is designed to be fired to intercept other Missiles. The cost would be the same as the cost of a regular Missile. The Anti-Missile would automatically be consumed if an enemy fires a regular Missile at your nation during war, meaning you don't get hit. In addition, using an Anti-Missile means you can't fire a regular Missile for that day of war, or if you've already fired a Missile then you can't intercept one for that day. You can intercept one Missile per day for every war you are engaged in, so you can intercept multiple Missiles but intercepting just one means you can't fire an offensive one that day.
Also another idea: It can also take out Nuclear weapons, but the Nuke still detonates in the air doing some infra/radiation damage to your nation. That's how it would be balanced between being able to take out nukes.
Thoughts?