Citing security details he declassified for his speech, Bush described al-Qaida's burgeoning operation in Iraq as a direct threat to the United States. In derisive terms, Bush accused critics in Congress of misleading the American public by suggesting otherwise.
"That's like watching a man walk into a bank with a mask and a gun and saying, 'He's probably just there to cash a check,' " Bush told troops at South Carolina's Charleston Air Force Base.
Bush is up against highly skeptical audiences with 18 months left in office. The public has largely lost faith in the war, Congress is weighing ways to end it, and international partners have fading memories of the 2001 terrorist attacks against the U.S.
In broad strokes, Bush's approach links the Iraq war to an event that Americans remember deeply - the Sept. 11 attacks - and not the sectarian strife among Iraqis. That violent infighting among Iraqis has caused many in the U.S. to question the war's point.
Al-Qaida, led by Osama bin Laden, orchestrated the terrorist strikes on the U.S. by turning hijacked airplanes into killing machines. That was almost six years ago.
Now a fresh intelligence estimate warns that the United States is in a heightened threat environment, mainly from al-Qaida. The terror group is seizing upon its affiliate, al-Qaida in Iraq, to recruit members and organize attacks, the report found.
"I've presented intelligence that clearly establishes this connection," Bush said after spelling out details of foreign ties and leadership of al-Qaida in Iraq.
"Those who justify withdrawing our troops from Iraq by denying the threat of al-Qaida in Iraq and its ties to Osama bin Laden ignore the clear consequences," he said. "If we were to follow their advice, it would be dangerous for the world and disastrous for America."
Yet his critics argue just the opposite point - that the war is not reducing the threat to America, but increasing it by swelling and unifying al-Qaida's numbers.
Al-Qaida had no active cells in Iraq when the U.S. invaded in March 2003, and its operation there is much larger now than before the war, U.S. intelligence officers say.
The war itself has turned into a valuable recruiting tool for al-Qaida, senior intelligence officials concede. Bush denied that the war triggered al-Qaida's operations in Iraq.
For his setting, Bush chose Charleston Air Force Base, a vital launching point for cargo and military personnel headed to Iraq. He watched crates of supplies being loaded onto a C-17 at the base, which ships thousands of tons of cargo to front-line troops.
The White House emphasizes that al-Qaida leaders in Iraq have sworn allegiance to bin Laden, who has avoided capture. He is believed to be living in the rugged Pakistan terrain.
On any given day, determining the specific source of violence can be a murky endeavor.
The intelligence estimate did say that that international counterterrorism efforts since 2001 have hampered al-Qaida's ability to attack the United States again.
Accompanying Bush on Tuesday was Republican Sen. Lindsey Graham, a notable defender of the president's policies in Iraq. Such Republican support for Bush in the Senate has been eroding.
Still, Bush has rebuffed attempts to pull troops out of Iraq. He says he will not consider a change in strategy until receiving an updated military assessment in September.
The president came to this same military airlift hub nine months ago, when he touted plans for victory in Iraq in a campaign stop. After his speech Tuesday, he was returning to the White House to meet with Jordan's King Abdullah II, a key U.S. ally.