According to John McCain's website:
John McCain strongly supports the development and deployment of theater and national missile defenses. Effective missile defenses are critical to protect America from rogue regimes like North Korea that possess the capability to target America with intercontinental ballistic missiles, from outlaw states like Iran that threaten American forces and American allies with ballistic missiles, and to hedge against potential threats from possible strategic competitors like Russia and China. Effective missile defenses are also necessary to allow American military forces to operate overseas without being deterred by the threat of missile attack from a regional adversary.
For starters, north Korea doesn't possess ICBM capabilities. Second, it's hard to see how national missile defense will protect our forces from Iranian missile attacks when our forces are right next door in Iraq and Afghanistan. Indeed, it's unclear why we'd be particularly worried about any sort of ballistic missile attack given the close quarters situation at hand. But while this is a bit dishonest and ignorant, the business about hedging against "potential threats from possible strategic competitors like Russia and China." Simply put, a scenario in which the United States possesses an effective ability to shoot down a Russian or Chinese ICBM threat would be completely intolerable in Moscow or Beijing. It would, in effect, give the United States a viable a threat of a nuclear first strike.
Neither Russia nor China is going to let that happen. Instead, they'll spend money on building up their nuclear arsenals in order to maintain their deterrent capacity. Thus, at great cost to the Unites States, to Russia, and to China we'll be back at the status quo. But beyond the monetary cost, the large buildup in Chinese nuclear capabilities that would result from this situation would force India to engage in a nuclear build-up of its own. And that, in turn, would force Pakistan to follow suit. This large increase in the global stock of nuclear weapons would, of course, imply an increase in the odds of a nuclear accident or the loss or theft of nuclear material. At the same time, a nuclear buildup of this sort might create incentives for Iran to reinitiate its nuclear weapons research program. And even if it didn't, revitalizing the Non-Proliferation Treaty desperately requires the status quo nuclear powers to be working together on nuclear issues, and fulfilling our treat obligations to move toward reduced arsenals.
In short, what McCain has on tap here is a recipe for disaster -- a breakdown in great power relations, new arms races, massive nuclear proliferation, etc. And why? I suspect the last bit is the real reason. He wants "to allow American military forces to operate overseas without being deterred." Basically, we need to spend huge sums of money and encourage an enormous amount of nuclear proliferation because that would facilitate the launching of new aggressive wars. Probably the proliferation McCain's policies helped induce would become the rationale for a new round of warfighting.