Al Qaeda has shown great resilience, continuing even as top leaders have been killed or captured. Even with Osama Ben Laden in hiding, other militants have stepped forward to carry out his calls for terror. Hundreds of arrests in France, Britain, and elsewhere in Europe have thwarted plots but did not stop the attacks on transport systems in Madrid, Spain and London.
The applause that greeted the announcement from the Iraqi prime minister that "Zarqawi was eliminated" was reminiscent of the whoops and celebrations that Saddam Hussein's capture inspired in 2003.
But the spiralling violence in Iraq since then has shown that decisive or defining victories are elusive in a war as complex as the one against terror. It is possible that Zarqawi's Al Qaeda in Iraq will intensify its attacks just to show that his death did not cripple it.
"We should have no illusions. We know that they will continue to kill, we know that there are many, many obstacles to overcome," said British Prime Minister Tony Blair.
For Western intelligence agencies and troops on the front lines, Zarqawi's killing is an undoubted boost.
With his horror videos — including one that showed the beheading of American hostage Nicholas Berg — the Jordanian-born militant had seemed to revel in taunting those seeking catch or eradicate him. His bloodshed made the US-led war on terror look impotent to some extent, an impression that the air strike that killed him helped to dispel.
"It indicates that the intelligence services and police are now more capable of infiltrating the terror groups," said Italian expert Stefano Silvestri, president of the Institute of International Affairs in Rome.
Zarqawi's hands-on violence always made it likely that he would be caught out in the end. Unlike Ben Laden and his top deputy Ayman Zawahri, Zarqawi's focus seemed not so much to stay alive but to do as much damage as possible, through suicide attacks, kidnappings and killings that stole some of Ben Laden's thunder. Zarqawi's older brother said his family had long anticipated his death.
But some of his methods and efforts to ignite a civil war between Sunnis and Iraq's majority Shiites also made him a divisive figure among Iraqi resistance groups, and experts say his death could now provide them with an opportunity to try to close ranks.
Al Qaeda in Iraq described Zarqawi as a martyr. But Western experts said that because he made his name through brutality, rather than as an ideologist or as thinker, Zarqawi was not likely, in death, to become a widely respected and inspirational figure for militants.
John Leicester, The Associated Press