Poland and the United States struck a deal Thursday that will strengthen military ties and put an American missile interceptor base in Poland, a plan that has infuriated Moscow and sparked fears in Europe of a new arms race.

"We have crossed the Rubicon," Polish Prime Minister Donald Tusk said, referring to U.S. consent to Poland's demands after more than 18 months of negotiations.

Washington says the planned system, which is not yet operational, is needed to protect the U.S. and Europe from possible attacks by missile-armed "rogue states" like Iran. The Kremlin, however, feels it is aimed at Russia's missile force and warns it will worsen tensions.

U.S. officials also said the timing of the deal was not meant to antagonize Russian leaders at a time when relations already are strained over the recent fighting between Russia and Georgia over the South Ossetia region.

In an interview on news channel TVN24, Tusk said the United States agreed to help augment Poland's defenses with Patriot missiles in exchange for placing 10 missile defense interceptors in the eastern European country.

He said the deal also includes a "mutual commitment" between the two nations to come to each other's assistance "in case of trouble."

That clause appeared to be a direct reference to Russia, which has threatened to aim its nuclear-armed missiles at Poland — a former Soviet satellite — if it hosts the U.S. site.

Poland is year, NATO endorsed the U.S. plan to expand its global missile defense shield with the planned site in Poland and a linked radar tracking base in the Czech Republic.

"Only evil people should be afraid of our agreement," Sikorski told reporters after Rood and his Polish counterpart, Andrzej Kremer, initialed the agreement at the Foreign Ministry.

The U.S. has also reached an agreement with the Czech Republic's government to place the radar component of the missile defense shield in that country. That deal still needs approval from Czech parliament.