WASHINGTON  The United States and Poland reached a long-stalled deal on Thursday to place an American missile defense base on Polish territory, in the strongest reaction so far to Russia’s military operation in Georgia.

Russia reacted angrily, saying that the move would worsen relations with the United States that have already been strained severely in the week since Russian troops entered separatist enclaves in Georgia, a close American ally. At a news conference on Friday, a senior Russian defense official, Col. Gen. Anatoly Nogovitsyn, suggested that Poland was making itself a target by agreeing to host the anti-missile system. Such an action “cannot go unpunished,” he said.

The deal reflected growing alarm in a range of countries that had been part of the Soviet sphere about a newly rich and powerful Russia’s intentions in its former cold war sphere of power. In fact, negotiations dragged on for 18 months  but were completed only as old memories and new fears surfaced in recent days.

Those fears were codified to some degree in what Polish and American officials characterized as unusual aspects of the final deal: that at least temporarily American soldiers would staff air defense sites in Poland oriented toward Russia, and that the United States would be obliged to defend Poland in case of an attack with greater speed than required under NATO, of which Poland is a member.