It's not to win a war with missiles, but to convince the other side that it's not really in their interest to attack the United States. Defense Secretary Donald Rumsfeld didn't discount the idea of the so-called scarecrow option or scarecrow missile defense. He said that whatever the United States does, it will take into account what extent it might deter or dissuade potential adversaries from attacking the United States. The bottom line is the Pentagon is looking for options to deploy sooner rather than later. The United States still has deterrence as the bedrock of protecting itself from missile attack.
Any adversary knows that if they attack the United States, they are asking for real trouble.
And the United States is the strongest military power on the planet. And it's not so much that they're worried that a rogue state such as North Korea or Iraq or some other country is seriously considering an attack on the United States, but the argument that the Pentagon makes is that even the threat of that attack can change the way other people react, and change public opinion. And they want something to counter that threat, even if it never comes to pass. So again, like the previous Persian Gulf scenario, they don't want to be in the position in the future where the United States is unwilling or afraid to act because they're concerned some country might lob a missile at a major U.S. city. They want to have some protection against that.