WASHINGTON -- President Obama's foreign policy has employed a starkly different tone than George W. Bush's, emphasizing engagement and cooperation rather than go-it-alone confrontation. Even so, analysts of various political stripes hadn't seen many big differences on substance.

Until now.

Obama's decision Wednesday to scuttle a costly and technically challenged long-range missile-defense system in Europe marks his most significant reversal of a Bush foreign policy priority. It could change the dynamic of what has been an increasingly tense relationship between the U.S. and Russia, which viewed the Bush plans for missile defenses in Poland and the Czech Republic as a threat.

Bush's proposed missile shield, designed to counter intercontinental missiles from Iran, "was very much a signature initiative of theirs, both with respect to Central Europe and with respect to missile defense," says John Pike, director of GlobalSecurity.org, a defense think tank.

Speaking to reporters at the White House, Obama was careful to portray his decision as a revamping, not an abandonment, of European missile defense. He said he would replace the long-range system Bush envisioned — which had a spotty testing record — with a more reliable defense system aimed at countering what Obama called a more imminent threat from Iran's short-range missiles, which can travel up to 5,000 miles and potentially strike continental Europe.

Defense Secretary Robert Gates said the new plan still places systems in Poland, the Czech Republic and other countries in the region, though the details are still to be ironed out. The plan calls for a ship-based component and some ground-based interceptors designed to target shorter-range missiles that are less difficult to hit.

"Our new missile-defense architecture in Europe will provide stronger, smarter, and swifter defenses of American forces and America's allies," Obama said.

The move wasn't unexpected — Obama had ordered a review of the Bush program shortly after taking office, and outside experts had questioned its feasibility even before then. Even so, Obama's announcement drew heated criticism from Republicans, who accused the president of abandoning central European allies and caving in to Russia in a naive bid for diplomacy.

"Short-sighted and harmful to our long-term security interests," complained Senate Minority Leader Mitch McConnell, R-Ky.

Democrats praised the decision; Senate Armed Services Committee Chairman Carl Levin called it "a sound choice that will improve our security."

Central to the debate over Obama's decision on missile defense is how it will be greeted by Russia, which has been a patron and trading partner of Iran, the Islamic republic that has bedeviled U.S. foreign policy for the last 30 years.

The Iranian regime has an active nuclear energy program, and although U.S. intelligence reports say Iran is not developing an atomic weapon, a new report by the International Atomic Energy Agency concluded Thursday that Tehran has the ability to make a bomb. That news, reported by the Associated Press, also said Iran is on its way to developing a missile system able to carry a nuclear warhead.

Obama is trying to negotiate with Iran. But if those talks fail, his administration will seek to impose "crippling sanctions," in the words of Secretary of State Hillary Rodham Clinton.

Russia, whose president, Dmitry Medvedev, will meet Obama at the United Nations next week, is key to any such effort. So far, it has resisted further sanctions on Iran, which it supplies with weapons and other technology.

If Obama's move was designed to nudge Russia, early signs Thursday weren't encouraging.

Russian Foreign Minister Sergey Lavrov said in a speech before Obama's missile-defense announcement that Moscow will continue to oppose any new sanctions over Iran's nuclear program.

Medvedev, however, said after the announcement that Obama's action was a "responsible move."

Until now — leaving aside domestic issues with foreign policy implications, such as the treatment of terror detainees — Obama's foreign policy hasn't radically departed from that of his predecessor, though the rhetoric has been much different.

He has talked more about engaging adversaries, though no presidential summits have materialized. He has continued Bush's gradual withdrawal from Iraq and added troops to Afghanistan.

Pushing a so-called "reset" button in the USA's relationship with Russia has yielded little that is tangible. Polls in various countries show that Obama is far more popular abroad than Bush is, but that popularity has yet to translate into visibly better cooperation with U.S. policy goals.

Christian Brose, who was a speechwriter for Bush Secretary of State Condoleezza Rice, wrote recently on foreignpolicy.com: "Our NATO allies have passed on sending more troops to Afghanistan and on lifting restrictions on those already there ... India and China don't share any of Obama's enthusiasm for a climate change deal. ... Pakistan is still dysfunctional and supporting terrorism. Iran and North Korea are all middle fingers and no unclenched fists. ... Rarely has a U.S. administration been so well-liked, so eager to engage with others, and had so little to show for it."

Obama officials disagree, of course, but it's not just former Bush officials who hold that view: In July, Columbia University professor and liberal blogger Lincoln Mitchell wrote a post entitled, "Why Obama's Foreign Policy Looks So Much Like Bush's." He previously was a Democratic consultant.

Republicans, including Obama's 2008 opponent, Sen. John McCain of Arizona, have supported many, though not all, of the new president's foreign policy moves. On Thursday, though, they erupted in a chorus of criticism.

McCain called the decision "seriously misguided," while Florida's Ileana Ros-Lehtinen, ranking Republican on the House Foreign Affairs Committee, dubbed it "a policy of appeasement."

Marc Thiessen, a former Bush speechwriter now at the Hoover Institution, said the "disastrous decision" sends "a signal of weakness to Tehran ... and a terrible signal to Russia, that they can bully us into abandoning our friends in what they consider their sphere of influence."

The partisan uproar came despite the endorsement of the move by Gates, who was Bush's secretary of Defense when the deals with Poland and the Czech Republic were finished last year and who holds the same job for Obama.

"This new approach provides a better missile-defense capability for our forces in Europe, for our European allies and eventually for our homeland than the program I recommended almost three years ago," Gates told reporters at the Pentagon after Obama spoke.

The program Gates once supported has been controversial from the moment Bush proposed it in a speech on May 1, 2001, months before the 9/11 attacks and a year before revelations about Iran's nuclear program.

It called for putting 10 land-based interceptors in Poland and a ground-based radar in the Czech Republic.

The Europe system was never deployed, so it wasn't tested. But similar ground-based interceptors failed to hit targets in five of 13 tests, according to the Pentagon, and they have not demonstrated an ability to detect decoys, the Government Accountability Office says.

The Bush system would have cost $9 billion to $13 billion, according to the Congressional Budget Office, and still would have left parts of Europe unprotected from an Iranian missile. Instead of the long-range interceptors, the United States will put in place more seasoned technology that will focus on medium- and short-range missiles, of which Iran has hundreds, Gates said.

Critics of the Bush approach praised Obama's announcement.

"What the president is proposing here actually produces more defense sooner than the program it replaces," said former Pentagon testing chief Philip Coyle, a longtime skeptic of the Bush program.

"The canceled European deployment would have added only marginally and at high cost to the full coverage of the United States already afforded by the existing ground-based interceptors," physicist Richard Garwin, who helped design the hydrogen bomb and recently was on a commission to assess the ballistic missile threat, said in an e-mail.

In Central Europe, reactions to the news were mixed. Bush's plan had never been popular in the Czech Republic, where polls showed 70% opposed it. In a March, a Czech government fell in part because it supported the missile shield.

Still, it was seen in both countries as a bulwark against an aggressive and expansionist Russia.

"Much of Europe — especially the Central and Eastern regions — will now view the United States as unable to fulfill its promises to its allies in the face of a strengthening Russia," said an analysis by Stratfor, a Texas-based intelligence firm.

Obama's shift on missile defense may embolden the Russians and "encourage them to push other buttons," said Janusz Bugajski, director of the New European Democracies Project at the Center for Strategic and International Studies, a Washington think tank.

Bugajski said it did not go down well in Poland that Obama made the announcement on the 70th anniversary of the Soviet Union's invasion of Poland at the start of World War II.

He said the United States will need to take steps to reassure Central and Eastern European countries that they will be protected against Russia. For example, NATO could devise defense plans for the countries, he said, or station troops on their territories.

Obama and Medvedev plan to meet twice next week, once at the United Nations and again in Pittsburgh for the G-20 economic conference. Those meetings could show what dividends, if any, the president will reap from his first major foreign-policy shift from his predecessor.

Contributing: David Jackson and Tom Vanden Brook, wire reports.