The killing of terrorist leader Abu Musab al-Zarqawi is a positive development in the war in Iraq. However, the tempered remarks of President Bush, in comparison to the optimism that succeeded the capture of Saddam Hussein, appropriately conveyed the complexity of the war in Iraq.

Mr. al-Zarqawi, the leader of al-Qaida in Iraq, was killed Wednesday when U.S. planes dropped bombs on the house he was hiding in north of Baghdad. The Jordanian-born terrorist leader was the most wanted man in Iraq for his role in many of the most brutal attacks there including beheadings, kidnappings and suicide bombings. His killing overshadowed the announcement that the Iraqi parliament had agreed on an interior minister, a defense minister and a national security adviser, after months of negotiation between the Shiite-dominated government and the Sunni minority.

Mr. al-Zarqawi took credit for the November 2005 triple suicide bombings against hotels in Amman, Jordan, that killed 60 people and for the February 2005 suicide car bombing of a crowd of police and Iraqi National Guard recruits in Hillah, which killed 125 people. In May 2004, American Nicholas Berg was beheaded. A videotape of the atrocity identified Mr. al-Zarqawi as the killer.

American and Iraqi officials also believe he played a large part in the suicide bombings of the United Nations headquarters in Baghdad in 2003 and the bombing of the Al Askari shrine in February, which launched sectarian violence that has escalated.

“Zarqawi is dead,” President Bush said at the White House Thursday. “But the difficult and necessary mission in Iraq continues. We can expect the terrorists and insurgents to carry on without him.”

These comments were in stark contrast to the positive outlook the president gave in December 2003 when former Iraqi leader Saddam Hussein was captured.

“In the history of Iraq, a dark and painful era is over. A hopeful day has arrived. All Iraqis can now come together and reject violence and build a new Iraq,” the president said then. Since then the violence has worsened and sectarian splits have been exacerbated.

Last month, President Bush, along with British Prime Minister Tony Blair, was again asked it he had made mistakes in his handling of the war in Iraq. The president said he’d learned to eliminate the tough guy talk. Perhaps that is why he was more subdued about Mr. al-Zarqawi’s death. Or maybe, it is because even experts disagree on the significance of his death since recent killings in Iraq have been carried out by militias with no connection to al-Qaida in Iraq.

Still, it is significant that Mr. Zarqawi’s group was infiltrated by informants who notified Iraqi and U.S. forces to his location in time for airstrikes to be launched. Such infiltration is the best way to undermine a terrorist group, Bruce Hoffman of the Rand Corp. told The New York Times. “Information that sows internal discord and disloyalty has a radiating effect in the organization. That could set in motion the unraveling of al-Qaida,” he said, noting that suspicion of another and subsequent assassinations of senior leaders led to the demise of Abu Nidal’s terrorist organization in the 1980s.

In the meantime, the larger question of whether the Iraq war is being fought in a way that can produce success for the new Iraqi government as well as for the United States and Britain is far from answered.