Forget about the WMD, it was a smokescreen. The President knew it, the VP knew it, the Republicans knew it, and the Democrats knew it. It was a sales tool to get the world to accept the invasion of Iraq. WMD was one of several reasons for the attack. The others were complicated, difficult to sell to the American public and 'unmentionable in public. Iraq was the second stage of a long range plan to transform the Middle East and destroy Al Qaeda in the process.
We often think of foreign affairs in a simplistic terms. Someone hits us, we hit back. I don’t think we, as Americans, are well known for subtle diplomacy. However in this case it might help to take a longer view. It is not hard to think of the Middle East as a social and political sewer, where dictators and extremists control almost all aspects of society.
Yes, there are some exceptions, but not many. Most countries are breeding grounds for Muslims extremists. Iran is a growing problem. Saudi Arabia was the home of most of the hijackers and is the center of Wahabi Islam, the most extreme of the extremists. Syria supports terrorists and, at the time of 9/11, completely dominated Lebanon. Egypt practices a dictatorial "democracy" with a growing militant populace. Libya was developing nuclear weapons and supporting supporting terrorism, and Pakistan was a strong supporter of the Taliban. There is more, but you get the idea.
After deposing the Taliban, the U.S. faced a next-move problem, what to do? Bush and the neocons were not blind to history in Afghanistan. They had no intention of making the same mistake the British and Russians made. The U.S. wanted a friendlier government in Afghanistan, not necessarily a Western style democracy. Capturing bin Laden was less important than destroying the foundation of his support. Looking at the long term, the government, led by the White House, saw Iraq as the next logical step. They would eliminate Saddam, a truly bad guy, begin the transformation of the region by installing a democracy, assuming they were successful.