WARSAW, Poland—The United States and Poland signed a deal Wednesday to place a U.S. missile defense base just 115 miles from Russia—a move followed swiftly by a new warning from Moscow of a possible military response.

For many Poles—whose country has been a staunch U.S. ally—the accord represented what they believed would be a guarantee of safety for themselves in the face of a newly assertive Russia.

Negotiators sealed the deal last week against a backdrop of Russian military action in Georgia, a former Soviet republic turned U.S. ally, that has worried former Soviet satellites across eastern Europe. It prompted Moscow’s sharpest rhetoric yet over the system, which it contends is aimed at Russia despite Washington’s insistence the site is purely defensive.

After Wednesday’s signing, Secretary of State Condoleezza Rice dismissed any suggestion the 10 missile defense interceptors—which Washington says are intended to defend Europe and the U.S. from the possible threat of long-distance missiles from Iran—represent a threat to Russia.

She denounced an earlier threat from a Russian general to target NATO member Poland, possibly even with nuclear weapons, for accepting the facility.