FORT BLISS, Texas — President Barack Obama says the end of the U.S. combat mission in Iraq is not a time for a victory lap.

Speaking with troops in a dining hall on an Army base in El Paso, Texas, Obama said there is still much work to be done before Iraq can be an effective partner for the U.S. But he also said that Iraq has an opportunity to create a better future for itself.

Obama will formally announce the end of the combat mission Tuesday night in an Oval Office address to the nation. About 50,000 U.S. troops will remain there – in a training and backup role. Under an agreement signed by the two countries, all U.S. troops must be out of Iraq by 2011.

Before arriving at Fort Bliss at midday Tuesday, Obama telephoned former President George W. Bush, who ordered U.S. troops to invade Iraq and topple Saddam Hussein in 2003, a decision that Obama strongly opposed. Aides described the phone call from Air Force One as brief and declined to reveal what was said.

Ahead of his 8 p.m. EDT Oval Office address, Obama was visiting the sprawling Fort Bliss base in El Paso, Texas, an installation that has seen repeated troop deployments to Iraq. Some of its servicemen and women are among the fewer than 50,000 troops who remain in Iraq in a training and backup role.

The administration has called the change of mission in Iraq an important milestone in a long and divisive conflict, but not a cause for celebration.

“This is not a time for premature victory parades or self-congratulation, even as we reflect with pride on what our troops and their Iraqi partners have accomplished,” Defense Secretary Robert Gates told an American Legion audience in Milwaukee.

“I am not saying all is, or necessarily will be, well in Iraq,” he said, noting the continued violence and lack of a new government more than six months after national elections.

The combat mission in Iraq has left more than 4,400 U.S. troops dead and thousands more wounded.

Obama was an early critic of the war, speaking out against it during the U.S. invasion and promising during his presidential campaign to bring the conflict to an end. The White House sees Tuesday’s benchmark as a promise kept and has gone to great lengths to promote it as such, dispatching Vice President Joe Biden to Iraq to preside over a formal change-of-command ceremony and raising Tuesday night’s remarks to the level of an Oval Office address, something Obama has only done once before.

Among Obama’s goals on Tuesday is honoring those who have served in Iraq since the March 2003 invasion, many returning to the battlefield for multiple tours of duty. White House spokesman Robert Gibbs said Monday that while the Iraq war would have never happened had Obama been commander in chief at the time, the president holds the service and sacrifice of the troops in high regard.

Appearing on nationally broadcast interviews Tuesday morning, Gibbs repeatedly brushed aside questions about whether Obama would credit Bush’s troop surge with helping to pave the way for the withdrawal.

Top Republicans, however, were in no doubt. “Some leaders who opposed, criticized, and fought tooth-and-nail to stop the surge strategy now proudly claim credit for the results,” House GOP leader John Boehner said, in excerpts of a speech he was to give to the American Legion convention in Milwaukee. “Today we mark not the defeat those voices anticipated – but progress.”

In Gibbs’ appearances, he said it’s “not up for question” that candidate Obama agreed sending 30,000 more troops to Iraq would bolster security. But “a number of things” brought the United States to this point, including the move toward greatrer political accommodation among the Sunni, Shia and Kurdish factions, the spokesman said.

Pressed on this point, Gibbs said, “Again, I think the president has always stated, and always believed” that adding significant numbers of American troops would stabilize the security environment, “but obviously the leaders in Iraq had to make some political accommodation to move that nation forward.”

Asked if Obama would support sending combat troops back if new waves of violence threatened the country, Gibbs said that Obama had been assured recently by commander Gen. Ray Odierno that such a scenario would be very unlikely.

“This is not a victory lap,” he said. “You’re not going to see any ‘Mission Accomplished’ banners that will be unfurled. ”

Since the start of the war, 200,000 personnel from Fort Bliss have deployed to Iraq, serving in every major phase of the war. Fifty-one soldiers from the base died there and many more were wounded.

Last week, some 600 soldiers from the 1st Brigade Combat Team returned to the base as part of Obama’s self-imposed Aug. 31 deadline for having all U.S. combat troops out of Iraq. Just about 50,000 U.S. troops will remain, down from a peak of nearly 170,000 in 2007. U.S. troops will no longer be allowed to go on combat missions unless requested and accompanied by Iraqi forces.

Administration officials have been careful to avoid equating the end of the combat mission with a mission accomplished. That was the phrase on the now-infamous banner that flew on an aircraft carrier seven years ago when Bush declared an end to major combat operations in Iraq, a symbol the Bush White House came to deeply regret as the war dragged on.

Under a security agreement signed between the U.S. and Iraq before Obama took office, all U.S. forces must leave Iraq by the end of 2011. But the Obama administration insists the U.S. is not abandoning Iraq and is ramping up a diplomatic corps to help stabilize the country’s government and economy over the coming years.

“This redoubles the efforts of the Iraqis,” Gibbs said. “They will write the next chapter in Iraqi history, and they will be principally responsible for it. We will be their ally, but the responsibility of charting the future of Iraq first and foremost belongs to the Iraqis.”

Ahead of Tuesday night’s remarks, Obama also planned to speak with Bush. While Bush’s decision to invade Iraq was criticized by many, the troop surge Bush ordered in 2007 has been credited with tamping down violence in Iraq and helping keep the country from falling into a civil war.

Gibbs was interviewed on ABC’s “Good Morning America,” NBC’s “Today” show, CBS’s “The Early Show,” CNN, Fox News Channel, MSNBC and National Public Radio.