Britain and Denmark might refuse to allow the Pentagon to upgrade radars based on their territory that are critical to an anti-missile shield. The damage could be far broader. Allies might become much less likely to defer to Washington on issues of major concern because they believe that it no longer intends to return the favor. Unilateral withdrawal would trigger a divisive political debate at home. Members of Congress galvanized into opposing the administration's missile defense plans by a unilateral withdrawal would have the public behind them.
Polls show that support for missile defense is closely tied to the costs involved.
In a word, ideology. Nearly all its senior officials have dedicated their careers to attacking the ABM Treaty. This time, unlike during the Reagan and first Bush administrations, they might not let the chance go by to bury it once and for all. Compounding the problem is a diplomatic miscalculation. Key allies dropped their harsh public criticism of missile defense after Mr. Bush's election thinking that a less confrontational approach would enable them to temper a policy that they consider unnecessary and unwise.