WASHINGTON — President Bush on Thursday ordered an indefinite halt in U.S. troop withdrawals from Iraq after July, embracing the key recommendations of his top war commander. Bush said Gen. David Petraeus will "have all the time he needs" to consider when more American forces could return home.

Bush's decisions virtually guarantee a major U.S. presence in Iraq throughout his term in office in January, when a new president takes office.

In another major decision, the president announced he will seek to relieve the heavy strain on the Army by reducing the length of combat tours in Iraq and Afghanistan to 12 months, down from the current level of 15 months. He said the change would take effect on Aug. 1, and would not affect U.S. forces already deployed on the front lines.

Bush said U.S. forces have made major gains since he ordered a buildup of about 30,000 U.S. forces last year. "We have renewed and revived the prospect of success" the president said.

Bush delivered his remarks in the Cross Hall of the White House before an audience of veterans' service groups and Vice President Dick Cheney, Defense Secretary Robert Gates and Secretary of State Condoleezza Rice.

The president's decision had been foreshadowed by two days of testimony before a skeptical Congress by Petraeus and Ryan Crocker, the U.S. ambassador to Baghdad. Now in its sixth year, the war has claimed the lives of more than 4,000 U.S. troops and cost more than $500 billion.

Iraq and the sagging economy have taken a heavy toll on the public's view of Bush. His job approval rating has fallen to just 28 percent, a new low in the Associated Press-Ipsos poll.

Bush said the United States would proceed with planned drawdowns of U.S. forces, bringing home about 20,000 of the 30,000 troops he sent to Iraq last year to combat sectarian violence. The additional troops were also intended to help restore basic security and provide a sense of calm to allow Iraqi leaders to attempt to achieve political reconciliation.

"I've told him (Petraeus) he'll have all the time he needs," Bush said. "Some have suggested that this period of evaluation will be a pause. That's misleading, because none of our operations in Iraq will be on hold. Instead we will use the months ahead to take advantage of the opportunities created by the surge and continue operations across the board."

While acknowledging that "serious and complex problems remain in Iraq," Bush said that "a major strategic shift" has occurred since the buildup.

"Today we have the initiative," the president said.

At a Capitol Hill news conference, Senate Majority Leader Harry Reid and House Speaker Nancy Pelosi said Bush's announcement on reducing combat tours was long overdue but that keeping troops committed to Iraq was unacceptable.

Bush's speech "can only be described as one step forward and two steps back," Reid said.

Pelosi, D-Calif, said Bush failed to answer key questions including what conditions would allow troops to come home and the impact on U.S. ground forces.

"We need real answers from the commander in chief and the president of the United States," she said.

Reid, D-Nev., said the Senate would vote soon on legislation demanding that troops spend as much time at home as they spend in combat. Similar proposals have failed in the past, and Bush would likely veto such a measure because it would reduce the military's flexibility.

When asked to respond to Bush's challenge of a veto on the upcoming war spending bill if Democrats add a timetable for troop withdrawals, Reid said he couldn't help but smile because the threat was nothing new.

"We have a plan, and we will execute it, in spite of the president, and his usual, normal harangue, "take care of the troops," he said.

Democratic presidential contender Sen. Hillary Rodham Clinton, D-N.Y., said the American people have run out of time and patience on Iraq. She said the military buildup has failed to yield political progress in Iraq, and that if elected, she will end the war responsibly. "It's time for the president to answer the question being asked of him: In the wake of the failed surge, what is the endgame in Iraq?"

Her Democratic rival, Sen. Barack Obama, D-Ill., said Bush's announcement means 140,000 troops will be in Iraq until the end of the year.

"In other words there is no end in sight under the Bush policy," he told a few thousand people at Theodore Roosevelt High School in Gary, Ind.

Obama also said that Bush had to reduce the length of rotations because the military had no other choice. "The Army was saying it can't sustain the rotation these people are on," he said.

House Armed Services Committee Chairman Ike Skelton, D-Mo., said he was glad Bush had decided to shorten active duty deployments because the longer tours, he said, have caused good soldiers to leave the Army. But he said he did not think the step went far enough to repair the readiness problems facing the Army.

Bush also called on Congress to send him a spending bill for Iraq that does not include any timetables for troop withdrawals or exceed the $108 billion he has requested. Last spring, Congress added $17 billion in unrequested domestic add-ons such as children's healthcare, homeland security and heating subsidies.

Now, Democrats are eyeing using this year's war funding bill to stimulate the economy with road-building funds, additional unemployment benefits, a summer jobs program and additional food stamp benefits. The measure is slated to advance later this spring.

Bush said he would veto the spending measure if Congress fails to meet his conditions.

"While this war is difficult, it is not endless," Bush said in a message directed to troops, but surely to the American public as well.

The president said that only as conditions in Iraq improve will he bring more troops home, a policy he calls "return on success."

"The day will come when Iraq is a capable partner of the United States," Bush said. "The day will come when Iraq's a stable democracy that helps fight our common enemies and promote our common interests in the Middle East.

"And when that day arrives, you'll come home with pride in your success," Bush said to the military and U.S. civilians in Iraq.

Bush used his speech to challenge Iran anew. He said the regime in Tehran has a choice to make: live in peace with its neighbor, or continue to fund and train militant groups that terrorize Iraqi people — charges that the Tehran government denies.

"If Iran makes the right choice, America will encourage a peaceful relationship between Iran and Iraq," Bush said. "Iran makes the wrong choice, America will act to protect our interests, and our troops, and our Iraqi partners."

After his remarks, Bush boarded a helicopter on the South Lawn to begin a journey to Texas for a weekend at his ranch.

In remarks prepared for a Senate hearing later on Thursday, Gates said he and the senior military leadership at the Pentagon "concur with the course the president has chosen in Iraq."

"I do not anticipate this period of review to be an extended one, and I would emphasize that the hope is, conditions on the ground will allow us to reduce our presence further this fall," Gates said. "But we must be realistic. The security situation in Iraq remains fragile and gains can be reversed."