Robert Farley observes:
For the last two years, we have justified putting a missile defense system in Eastern Europe explicitly around the threat of Iranian ballistic missiles. In addition to the extraordinary financial costs, this project has resulted in increased Russian hostility to the United States and to Russia's neighbors. And are we now to believe that this expensive and unpopular system is justified by the need to protect Poland from Iranian ballistic missiles armed with conventional warheads?
Naturally, though, the exorbitant financial cost of the program counts as a point in its favor. The US would never want to build something cheap, useless, and incredibly damaging to our relationship with Russia. But since the missile defense initiatives are so damn costly, they're also incredibly profitable to the people who build them, and thus to the members of congress who get their campaign contributions and to the think tankers who they support. The best way to kill this initiative would be a scientific breakthrough that allowed its goals to be achieved cheaply and with some efficacy. If that was on hand, diplomatic considerations just might win out.