If the United States builds a national missile defense system, it will be in violation of the Anti-Ballistic Missile Treaty, agreed to with the Soviet Union in 1972 and arguably still binding on Russia. It can either amend the treaty (which Russia has said it will not agree to) or withdraw from it. How does a country withdraw from a treaty? And is there a difference between withdrawing from a treaty and abrogating one?
The terms "withdraw" and "abrogate" are sometimes used interchangeably, but they don't mean the same thing. One side can legally withdraw from a treaty as long as it meets whatever conditions for withdrawal are set out in the treaty. To abrogate means to pull out without meeting those conditions, or to pull out in certain cases when the treaty does not set conditions for withdrawal.
The ABM treaty includes a specific and fairly simple withdrawal provision: If either party decides that changing circumstances have rendered the treaty contrary to its interests, it can withdraw six months after giving notice. The decision can be unilateral. The U.S. would not need Russia's permission, and Russia would have no legal recourse, even if it disagreed with the formal statement the U.S. would be required to issue.
Were the U.S. to abrogate the treaty, the legal effect would be more or less the same as if it withdrew. Russia could take its case to the U.N. International Court of Justice, but since there is no enforcement mechanism for international political treaties (unlike some economic treaties), the U.S. could not be punished even if found guilty. However, abrogating the treaty would have more serious diplomatic consequences than withdrawing from it, and the U.S. is unlikely to do so because it can withdraw from the treaty simply by issuing a statement.
The U.S. rarely withdraws from international treaties. The last time it did so was in 1978, when President Carter withdrew from the Mutual Defense Treaty with Taiwan in order to recognize China. In the 19th century, the U.S. regularly violated Indian treaties.