WASHINGTON  President Bush used the fifth anniversary of the start of the war in Iraq on Wednesday to make the case for persevering in a conflict that could have many more anniversaries. Democrats accused him of lacking a strategy to win and withdraw.

Mr. Bush, speaking before members of the armed forces and defense officials at the Pentagon, said in his frankest acknowledgment yet that the costs of the war, in lives and money, had been higher and longer lasting than he had anticipated.

But he remained unwavering in his insistence that the invasion of Iraq, which began in March 2003, had made the world better and the United States safer.

“Five years into this battle, there is an understandable debate over whether the war was worth fighting, whether the fight is worth winning, and whether we can win it,” he said. “The answers are clear to me. Removing Saddam Hussein from power was the right decision, and this is a fight that America can and must win.”