Bush SHOULD face investigation. He caused more needless suffering than Truman or LBJ.
That's why it's so ridiculous to say that Congress "voted to invade." It shifts the responsibility for the decision; it takes accountability away from where it should be. I don't give Congress a pass, but people forget why they voted to give Bush such authority. It was a show of unity, to demonstrate to Saddam that Americans weren't divided on this, and force his hand on inspections.
Yes, most knew that Bush would abuse the new power Congress gave him, and do the wrong thing. But that doesn't change the fact that it was Bush who abused the power, and did the wrong thing. No one forced him to invade. Many begged him not to. He was reckless, and stupid, and yes - criminal.
The very least we owe history is to hold him accountable for his actions - not characterize him as helpless to the whims of Congress. Congress never "voted to invade."