Seeing as how I'm too young to remember any of this, I'll just have to trust Jack Hitt's account of why Newt Gingrich decided to revive missile defense in the mid-1990s, after the program had been mocked to near-death in the 1980s:

[T]here was a time, not so long ago, when it was hard to come up with a good, intimidating national-security issue. Gingrich believed that the Democrats' skepticism of missile defense would serve as the key issue to flip the White House in 1996. Missile defense failed that test—but it didn't matter. The shield had been reborn as a hot-blooded, Republican-versus-Democrat wedge issue.

A wedge issue, mind you, whose annual bill comes to $11 billion—"a sum almost four times larger than the U.S. government's total spending on energy research." And, as Hitt documents in his piece, it's still very much a boondoggle. George W. Bush's chief contribution to said boondoggle, it turns out, was to order the Defense Department, back in 2002, to stop doing so many tests and just deploy the thing already. Suffice to say, that hasn't turned out well. In any case, plenty of scientists think the system will never work (and, even if it does, won't be worth the cost), but I thought this passage got at the darker heart of the matter:

Missile defense exists in a world of its own. It has a special budget process that exempts it from most congressional oversight, and it is pioneering a new acquisitions process that redefines the very nature of what constitutes a "threat." The system has a separate definition to denote what it means for a weapon to "work" and even what it means to "know" something to be true. The shield operates beyond the world of empirical testing, and outside the four service branches of the U.S. military. ...

It is America's Pyramid of Giza, our Colossus of Rhodes, our Great Wall—an infinitely advancing "system of systems" that, by the Pentagon's own description, can never be completed. It both works (in part or in theory) and does not work (as a whole or in practice). There is not, and never will be, a finished product. In time, the shield will shroud America and her allies, and a perpetual commitment to its everlasting need for further refinements and add-ons will be required to keep it functioning.

The italicized sentence seems spot-on. Back in 1995, the intelligence community declared that there wasn't any immediate need for missile defense, because no countries would be able to threaten the United States with ballistic missiles for at least 15 years. Wrong answer. So Republicans in Congress set up a commission, led by Rumsfeld and Paul Wolfowitz, to draw up a new threat assessment, based not just on "likely" threats, but on "possible" threats. Maybe a country with Scud technology could convert that into ICBM capability. Maybe Venezuela will threaten us with missiles. And so on.

Obviously, once you start thinking this way, missile defense can expand forever. Hitt meets with missile-defense contractors who give presentations on how to adapt the program to defend against possible space aliens. Sure, why not? Aliens, Venezuela, it's a dangerous world out there. And if there aren't enough enemies, we can make new ones. Not to mention the fact that missile defense can spur other countries to expand and modernize their arsenals, since that's the easiest way to beat the system. Indeed, apart from enriching contractors, undermining arms-control treaties seems to be the only thing missile defense has actually accomplished.

Just think, for a quarter of the cost we could have doubled what the government spends on energy research. Ah, but that would be wasteful, Republicans might say. After all, the consequences of global warming are pretty uncertain.