(CNN) -- Five years after he green-lighted the war in Iraq, President Bush will mark the anniversary by calling the debate over the conflict "understandable" but insisting that a continued U.S. presence there is crucial.

"The answers are clear to me," Bush says, according to excerpts of his speech to be delivered at the Pentagon on Wednesday, the day the war began in 2003.

"Removing Saddam Hussein from power was the right decision, and this is a fight America can and must win."

Almost 4,000 American troops have died in the war, a painful toll that Bush acknowledges in his remarks.

"No one would argue that this war has not come at a high cost in lives and treasure, but those costs are necessary when we consider the cost of a strategic victory for our enemies in Iraq."

Bush contends that the troop surge he ordered in January 2007 has been a success and was necessary at a point when "the fight in Iraq was faltering."