On the fifth anniversary of the start of the Iraq War, US President George W. Bush warned that a precipitous withdrawal could embolden Iran and extremist groups, indicating he was unwilling to significantly cut America's troop presence there during his last 10 months in office, despite urging from critics. "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," Bush said during a speech at the Pentagon on Wednesday. In such a scenario, he continued, "Iran would be emboldened as well - with a renewed determination to develop nuclear weapons and impose its brand of hegemony across the Middle East. Our enemies would see an America, an American failure in Iraq as evidence of weakness and a lack of resolve." Bush began his remarks by noting several questions looming large for Americans, who have watched almost 4,000 of their countrymen die in Iraq and a projected $1 trillion devoted to the war effort abroad as the domestic economy sputters. "There is an understandable debate over whether the war was worth fighting, whether the fight is worth winning, and whether we can win it," he said. "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 highlighted the end of Hussein's cash payments to the families of Palestinian suicide bombers as one of the many rationales - including ending the Iraqi leader's aggression toward his neighbors and torture of his own people - for his assertion. Though Bush was addressing US Defense Department officials and soldiers, his words were also aimed at his would-be successors in the Democratic Party, who have criticized his Iraq policy and called for the return of American troops. Barack Obama, the Illinois senator who narrowly leads New York Sen. Hillary Clinton in Democratic delegates and primary votes, gave a speech Wednesday in which he declared, "When I am commander-in-chief, I will set a new goal on day one: I will end this war. Not because politics compels it. Not because our troops cannot bear the burden - as heavy as it is. But because it is the right thing to do for our national security, and it will ultimately make us safer." "In order to end this war responsibly, I will immediately begin to remove our troops from Iraq," Obama pledged, according to his prepared remarks. In addition to stressing the contrast between his position and Bush's, Obama focused on Sen. John McCain, the presumptive Republican presidential nominee. "It is time to have a debate with John McCain about the future of our national security," Obama said. "The way to win that debate and to keep America safe is to offer a clear contrast, and that's what I will do when I am the nominee of the Democratic Party - because since before this war in Iraq began, I have made different judgments, I have a different vision, and I will offer a clean break from the failed policies and politics of the past." McCain's campaign counter-attacked in a statement put out by senior adviser Mark Salter. "John McCain wants American forces to come home when our clear and serious interests at stake in Iraq, which nearly 4,000 Americans have given their lives to secure, are truly safe; when al-Qaida is defeated; Iran's influence is contained; and the potential for a truly cataclysmic civil war in Iraq is remote," Salter said. "That, I think, is what is called 'making us safer.' Senator Obama's plan, if it can be charitably described as one, would do the reverse." Two days earlier, Clinton also weighed in on Iraq, declaring it was "time to end this war as quickly and responsibly as possible." She said, "Senator McCain and President Bush claim withdrawal is defeat. Well, let's be clear: withdrawal is not defeat. Defeat is keeping troops in Iraq for 100 years. Defeat is straining our alliances and losing our standing in the world. Defeat is draining our resources and diverting attention from our key interests. Now, withdrawal is not risk-free, but the risks of staying in Iraq are certain. And a well-planned withdrawal is the one and only path to a political solution." Despite the stance of the Democratic candidates, anti-war demonstrators had planned a march Wednesday evening on the National Democratic Committee office in Washington, as part of a host of five-year-anniversary protests. Earlier in the day, the activists protested outside the Internal Revenue Service offices and major media outlets, with dozens arrested for blocking entrances. Also on Wednesday, a Web site with close connections to al-Qaida said Osama bin Laden would be issuing a new message right around the fifth anniversary of the US-led invasion of Iraq. "Urgent, very soon by the will of God," read the announcement of the new message. "The response is what you see and not what you hear, by the warrior sheikh, Osama bin Laden." The message would be bin Laden's first for 2008 and follow up an hourlong, late December audio missive in which he warned Iraq's Sunni Arabs against fighting against al-Qaida in Iraq and vowed new attacks on Israel. Bin Laden's new message would come as it appears that al-Qaida's Iraqi branch is losing ground, despite still being able to carry out large-scale bombings. AP contributed to this report.