WASHINGTON– President Bush says he has no doubts about launching the unpopular war in Iraq despite the ”high cost in lives and treasure,” arguing that retreat now would embolden Iran and provide al-Qaida with money for weapons of mass destruction to attack the United States.

Bush is to mark the fifth anniversary of the U.S.-led invasion of Iraq on Wednesday with a speech at the Pentagon. Excerpts of his address were released Tuesday night by the White House.

At least 3,990 members of the U.S. military have died since the beginning of the war in 2003. It has cost taxpayers about $500 billion and estimates of the final tab run far higher. Nobel Prize-winning economist Joseph E. Stiglizt and Harvard University public finance expert Linda Bilmes have estimated the eventual cost at $3 trillion when all the expenses, including long-term care for veterans, are calculated.

Democrats offered a different view from Bush’s.

”On this grim milestone, it is worth remembering how we got into this situation, and thinking about how best we can get out,” said Rep. John Dingell, D-Mich. ”The tasks that remain in Iraq– to bring an end to sectarian conflict, to devise a way to share political power, and to create a functioning government that is capable of providing for the needs of the Iraqi people are tasks that only the Iraqis can complete.”

In his remarks, Bush repeated his oft-stated determination to prosecute the war into the unforeseen future.

”The successes we are seeing in Iraq are undeniable, yet some in Washington still call for retreat,” the president said. ”War critics can no longer credibly argue that we are losing in Iraq, so now they argue the war costs too much. In recent months, we have heard exaggerated estimates of the costs of this war.

”No one would argue that this war has not come at a high cost in lives and treasure, but those costs are necessary when we consider the cost of a strategic victory for our enemies in Iraq,” Bush said.

Bush has successfully defied efforts by the Democratic-led Congress to force troop withdrawals or set deadlines for pullouts. It is widely believed he will endorse a recommendation from Gen. David Petraeus, the top U.S. commander in Iraq, for no additional troop reductions, beyond those already planned, until at least September.

The U.S. now has about 158,000 troops in Iraq. That number is expected to drop to 140,000 by summer in drawdowns meant to erase all but about 8,000 troops from last year’s buildup.

”If we were to allow our enemies to prevail in Iraq, the violence that is now declining would accelerate and Iraq could descend into chaos,” Bush said. ”Al-Qaida would regain its lost sanctuaries and establish new ones fomenting violence and terror that could spread beyond Iraq’s borders, with serious consequences to the world economy.

”Out of such chaos in Iraq, the terrorist movement could emerge emboldened with new recruits … new resources … and an even greater determination to dominate the region and harm America,” Bush said in his remarks. ”An emboldened al-Qaida with access to Iraq’s oil resources could pursue its ambitions to acquire weapons of mass destruction to attack America and other free nations. Iran could be emboldened as well with a renewed determination to develop nuclear weapons and impose its brand of hegemony across the broader Middle East. And our enemies would see an American failure in Iraq as evidence of weakness and lack of resolve.”

Looking back, Bush said, ”Five years into this battle, there is an understandable debate over whether the war was worth fighting … whether the fight is worth winning … and whether we can win it. The answers are clear to me: Removing Saddam Hussein from power was the right decision and this is a fight America can and must win.”

Bush said the past five years have brought ”moments of triumph and moments of tragedy,” from free elections in Iraq to acts of brutality and violence.

”The terrorists who murder the innocent in the streets of Baghdad want to murder the innocent in the streets of American cities. Defeating this enemy in Iraq will make it less likely we will face this enemy here at home,” Bush said.

Bush said anew that the war was faltering a little more than a year ago, prompting him in January 2007 to order a big troop buildup known as the ”surge.”

”The surge has done more than turn the situation in Iraq around; it has opened the door to a major strategic victory in the broader war on terror,” he said.

”In Iraq, we are witnessing the first large-scale Arab uprising against Osama bin Laden, his grim ideology, and his terror network. And the significance of this development cannot be overstated ,” the president said.

”The challenge in the period ahead is to consolidate the gains we have made and seal the extremists’ defeat. We have learned through hard experience what happens when we pull our forces back too fast– the terrorists and extremists step in, fill the vacuum, establish safe havens and use them to spread chaos and carnage.”