WASHINGTON — During the G8 meeting in Genoa, U.S. President George W. Bush and his Russian counterpart Vladimir Putin agreed to start discussions on how the 1972 Antiballistic Missile Treaty might be modified or replaced.

Once they separated, however, Bush reiterated his ironclad determination to move ahead with an ambitious missile defense program regardless of Russian views, and Putin again emphasized his commitment to preserve the existing treaty.

Regardless of their motives, negotiations will now ensue. It is important that we clearly assess the pros and cons of various possible approaches to these negotiations. Three basic options stand out.