Bush apparently chose not to attack Zarqawi before the Iraq war in order to have a terrorist target to justify the war

This is an amazing story. Thanks to Jacob T. Levy in The Volokh Conspiracy for keeping it alive.

Avoiding attacking suspected terrorist mastermind

"NBC News has learned that long before the war the Bush administration had several chances to wipe out [Zarqawi's] terrorist operation and perhaps kill Zarqawi himself but never pulled the trigger.

In June 2002, U.S. officials say intelligence had revealed that Zarqawi and members of al-Qaida had set up a weapons lab at Kirma, in northern Iraq, producing deadly ricin and cyanide.

The Pentagon quickly drafted plans to attack the camp with cruise missiles and airstrikes and sent it to the White House, where, according to U.S. government sources, the plan was debated to death in the National Security Council.

...

Military officials insist their case for attacking Zarqawi’s operation was airtight, but the administration feared destroying the terrorist camp in Iraq could undercut its case for war against Saddam. [Emphasis added]"