Annan, in an interview with The Times of London, said he wonders what would have happened if Blair had told former President George W. Bush he would not go to war without a second U.N. resolution.
"I really think it could have stopped the war," he said. "It would have given the Americans a pause. It would have given them a very serious pause to think it through. ... All this would have raised a question: 'Do we go this alone?'"
Annan is promoting his memoirs, "Interventions -- A Life in War and Peace," The Guardian said. He wrote the book with Nader Mousavizadeh, who served as his special assistant at the United Nations.
In the book, Annan describes a 2005 conversation with Blair about the conflict between Israel and the Lebanese group Hezbollah. He says Blair perceived "a meta-conflict between modernity and the medieval, between tolerant secularism and radical Islam."
"This was not the Blair with whom I had agreed so passionately about the moral necessity of a humanitarian intervention to halt the Serbian attacks on the Kosovar Albanians in 1999 ... Something had changed in Blair, and with it, I felt, his ability to act as a credible mediator," he said.