President Bush suggested Friday that history will vindicate his decision to invade Iraq, saying he believed that a half century from now, it will be regarded as important a transition for the world as the democratization of Japan was after World War II.

"I'm absolutely convinced that some day, 50 or 60 years from now, an American president will be speaking to an audience saying, 'Thank goodness a generation of Americans rose to the challenge and helped people be liberated from tyranny,' " Mr. Bush said. " 'Democracy spread and the world is more peaceful for it.' "

During last year's campaign, Mr. Bush often spoke of his friendship with Prime Minister Junichiro Koizumi of Japan, remarking that a bitter enemy that Mr. Bush's father fought against in World War II has become a close friend and ally. He expanded on the theme during his recent trip in Asia, with Mr. Koizumi at his side, and used it again today to argue that history would prove him right in deciding to invade Iraq.

"Something happened between the time that my dad and your relatives signed up in World War II and I'm talking peace with Koizumi," he said. "And what happened was, Japan became a democracy."

Many outside experts and even some of Mr. Bush's own aides question his reliance on the comparison, noting that Japan was a unified state before World War II, but that Iraq has always been divided along religious and regional lines.

"It may sound too simple, but this is a comparison the president believes in deeply," one of his senior aides said when Mr. Bush was in Asia, declining to be quoted by name in discussing the president's thinking. "It's the argument he knows his presidency will be judged by."