Calling it a "relic of the Cold War," U.S. President George W. Bush is planning to tell Russia that the United States will pull out of the Anti-Ballistic Missile Treaty.
The 1972 treaty was an agreement between the two countries not to develop missile defence systems. It's been credited with close to 30 years of nuclear stability.
Bush said the Sept. 11 terrorist attacks were the final argument for scrapping the treaty.
He said Tuesday the attacks on Washington and New York "made it even more clear that we need to build limited and effective defences. We must protect America and her friends against all forms of terror, including the terror that could arrive on a missile."
Bush wants to conduct nuclear tests outlawed by the treaty.
The decision to withdraw comes after Secretary of State Colin Powell failed to bridge differences during meetings in Russia this week.
Either side has the legal right to withdraw from the treaty after giving six months' notice. Bush is expected to formally inform Russian President Vladimir Putin of the American decision in the next several days.
Putin has expressed concern at the idea of a U.S. withdrawal. He says it could jeopardize more than 30 years of work and defence negotiations.
Some U.S. allies in Europe think abandoning the treaty is an unwise move.
So do some American politicians. Sen. Joseph Biden, a Democrat and head of the Foreign relations Committee, said walking away from a treaty "that has helped keep the peace for the last 30 years ... would be a serious mistake."