Ten years after the invasion of Iraq, most Americans appear to believe that the war was not worthwhile. Perhaps they are correct, but we cannot know for sure because we do not know what would have happened. Consider, for example, the run-up to World War II. Churchill believed that the time when Hitler could have been stopped without a major war would have been when Germany re-militarized the Rhineland in 1936, as that was the last time when the combination of France & Britain enjoyed clear military superiority over Germany. But suppose that Churchill had been Prime Minister in 1936, that he had sent troops into the Rhineland to stop Hitler, and that the war had not been quick or easy. Suppose that the move had brought down the Nazi regime, but that a guerrilla army with Soviet support had arisen to fight the invading allies. Suppose it had taken the allies had struggled with the guerrillas for a decade, and had lost thousands, or tens of thousands, or even hundreds of thousands of troops in the effort. And even assume that the end result was a German government aligned with the Soviets. From our vantage, that result would still be enormously better than what actually did happen. But if that scenario had actually occurred, no one would have known that anything like World War II or the Holocaust would have ensued. Churchill most likely would have gone down as a blunderer who started a pointless and bloody war and failed to meet his objectives.
Nor do we know what would have happened without our intervention in Iraq. But we can speculate. Would we still be enforcing strict sanctions that were blamed for starving millions of Iraqis? Would we still be patrolling "no fly zones" in which our planes were fired upon on a weekly basis? Would we still have sizable military bases in Saudi Arabia to deter the Saddam regime? Recall that those three ongoing actions were on the short list of reasons bin Laden cited for the 9/11 attacks - they certainly provoked considerable anti-Americanism in the Arab and Muslim world.
And lets suppose that we had ended the sanctions and no-fly zones. Would we have been able to retain any leverage to keep Saddam from re-igniting his nuclear program? Would bin Laden have eventually found refuge or logistical support from a re-armed Iraq? Would Saddam be fomenting terrorism against us or Israel? Or threatening its neighbors? Would he be regarded throughout the Middle East as a hero who beat the Americans? No one really knows.