Washington ? President Bush said Saturday that the United States “will not run” from Iraq, directly challenging a prediction that U.S. officials have attributed to al-Qaida’s No. 2 leader, who said American power in Iraq may rapidly collapse, as it did in Vietnam when the U.S. “ran and left their agents,” creating a governmental vacuum.

In his weekly radio address, the president took the unusual step of replying to statements from a letter seized in military operations in Iraq that American intelligence officials said was from Ayman Zawahiri to Abu Musab al-Zarqawi, the terrorist commander in that country.

U.S. intelligence officials, responding to questions about the letter’s authenticity, have said it has been verified by multiple agencies over a period of time.

The letter attributed to Zawahiri advises al-Zarqawi that “things may develop faster than we imagine” in Iraq, and recalling the U.S. experience in Vietnam, it says, “We must be ready, starting now before events overtake us” to prepare for the departure of American forces.

Bush’s response Saturday was, “The al-Qaida letter points to Vietnam as a model. … Al-Qaida believes that America can be made to run again. They are gravely mistaken. America will not run, and we will not forget our responsibilities.”