For many Poles - whose country has been a staunch U.S. ally in Iraq and Afghanistan - 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.
"Missile defense, of course, is aimed at no one," Rice said. "It is in our defense that we do this."
She denounced an earlier threat from a Russian general to target NATO member Poland, possibly even with nuclear weapons, for accepting the facility.
Such comments "border on the bizarre, frankly," Rice told reporters in Warsaw. "The Russians are losing their credibility," she said, adding that Moscow would pay a price for its actions in Georgia, though she did not specify how.
"It's also the case that when you threaten Poland, you perhaps forget that it is not 1988," Rice said. "It's 2008 and the United States has a ... firm treaty guarantee to defend Poland's territory as if it was the territory of the United States. So it's probably not wise to throw these threats around."
Hours after the signing, Russia's Foreign Ministry warned that Moscow's response would go beyond diplomacy. The system to be based in Poland lacks "any target other than Russian intercontinental ballistic missiles," it said in a statement, contending the U.S. system "will be broadened and modernized."
"In this case Russia will be forced to react, and not only through diplomatic" channels, it said without elaborating.
Michael Mandelbaum, a foreign policy expert at Johns Hopkins University, told CBS News national security correspondent David Martin that placing missile defenses so close to its territory is a red line for Russia.
"No deployment of ballistic missile defense systems in their neighborhoods, at least not without their cooperation," Mandelbaum said.
Another red line: the two former Soviet republics of Ukraine and Georgia must not be allowed to join the NATO alliance, Martin adds.
"The Russian attitude is when we had a good reputation, we were pushed around by the West so maybe an edgy reputation, in the Russian view, isn't such a bad thing," Mandelbaum said.
The Center for Arms Control and Non-Proliferation in Washington criticized the deal, saying the U.S. missile interceptors are technologically unproven and will only confirm Russian suspicions the system is directed against Moscow and not at Iran.
The deal follows an earlier agreement to place the second component of the missile defense shield - a radar tracking system - in the neighboring Czech Republic, another formerly communist country now in NATO.
"We have achieved our main goals, which means that our country and the United States will be more secure," Polish Prime Minister Donald Tusk told Rice after the signing.
Many Poles agreed. "After what happened in Georgia, I believe that this is good protection for us," said Kazimierz Dziuba, 49, a hospital worker in Warsaw.
The Georgian conflict "made the Americans agree to this deal sooner because the Russians are getting too bossy," Dziuba said.
Not all Poles were happy, however.
Alina Kesek, an 82-year-old retired office clerk who lived through World War II, when Nazi Germany and the Soviet Union divided Poland between them, and then experienced four decades of Moscow-dominated communist rule, said the Patriot missiles were a "kind of provocation" toward Russia.
"This means a threat from the Russian side," said Kesek. "I am not very pleased with this deal."
Some residents in the northern Polish town of Redzikowo, where the missile defense facility will be located, fear it may expose them to retaliatory attacks or other dangers.
Along with the main deal, the two nations signed a so-called "declaration on strategic cooperation," which is to deepen their military and political partnership.
It includes a mutual commitment to come to each other's assistance immediately if one is under attack - enhancing existing obligations both have as NATO members.
The declaration also was accompanied by a promise from the U.S. to help modernize Poland's armed forces and to place a battery of Patriot missiles there by 2012.
Rice said the deal "will help both the alliance and Poland and the United States respond to the coming threats."
Poland and the United States spent a year and a half in formal talks, which snagged in the final phase on Poland's demands for the Patriot missiles and other points.
However, the deepening U.S.-Polish friendship dominated Wednesday's proceedings.
"In troubled times the most important thing is to have friends," Rice said. "But it is more important to have friends who share your values and your aspirations and your dreams. And Poland and the United States are those kind of friends."
Approval for the missile defense sites is still needed from the Czech and Polish parliaments. No date has been set for lawmakers in Warsaw to vote, but the deal enjoys the support of the largest opposition party as well as of the government.