US President George W Bush has said too much is at stake in Iraq for politicians to make "false charges" about the reasons for going to war.
Bush said the US military had a duty to "complete the mission"
Amid new questions in Congress about the intelligence used to justify the invasion, he said it was "irresponsible to rewrite history".
He told US veterans Iraq was now the central front in the "war on terror" and the US would prevail.
An opinion poll suggests less than half Americans support his foreign policy.
Flanked by veterans, Mr Bush was speaking at the Tobyhanna Army Depot, Pennsylvania, on Veterans' Day, a few weeks after the US death toll in Iraq passed the 2,000 mark.
Mr Bush said he was open to criticism for his policies but it was "deeply irresponsible to rewrite the history of how the war began".
"Some Democrats and anti-war critics are now claiming we manipulated the intelligence and misled the American people about why we went to war," he said.
"The stakes in the global war on terror are too high and the national interest is too important for politicians to throw out false charges."
More than 100 Democrats in Congress, he argued, had viewed the same intelligence about WMD and had voted to back military action.
Democrats are pushing for an investigation into why WMD were never found in Iraq despite intelligence claims before the war.
Senate Democratic leader Harry Reid of Nevada said this week Americans deserved to "get the truth about why the White House cherry-picked and leaked intelligence to sell the war in Iraq".
"The president may think this matter can be swept under the rug or pardoned away, but Democrats know America can do better," he added.
Warning to Syria
The time had come, Mr Bush said in his speech on Friday, for responsible Islamic leaders to denounce militants who distorted their religion.
Militants, he argued, aimed to build a "radical empire" and were enemies of Muslims and non-Muslims alike.
Listing some recent US security operations against militants in Iraq, he said: "We're on the hunt - we're keeping pressure on the enemy."
The best way to honour America's dead, he continued to loud applause, was to "complete the mission".
Mr Bush had strong words for Syria, saying it should "stop exporting violence and start importing democracy".
He called on Damascus to "stop trying to intimidate and destabilise" Lebanon, and to co-operate fully with the UN inquiry into the assassination of former Lebanese Prime Minister Rafik Hariri.
According to an AP-Ipsos opinion poll, 57% of Americans do not think the administration has high ethical standards and do not think Mr Bush is honest.
Only 42% approve of his handling of foreign policy and terrorism, the poll suggests.
The survey of 1,000 adults has a margin of sampling error of plus or minus 3%.
Mr Bush has recently suffered a series of political setbacks, from his failure to get Supreme Court nominee Harriet Miers approved to seeing vice-presidential aide Lewis "Scooter" Libby indicted over his conduct during an inquiry into the unmasking of a CIA agent.