Why does this not surprise me in the least? Even 400K wouldn't have been enough, and they knew it. In 1999, no less:

WASHINGTON -- The U.S. government conducted a series of secret war games in 1999 that anticipated an invasion of Iraq would require 400,000 troops, and even then chaos might ensue.

In its "Desert Crossing" games, 70 military, diplomatic and intelligence officials assumed the high troop levels would be needed to keep order, seal borders and take care of other security needs.
The documents came to light Saturday through a Freedom of Information Act request by the George Washington University's National Security Archive, an independent research institute and library.

"The conventional wisdom is the U.S. mistake in Iraq was not enough troops," said Thomas Blanton, the archive's director. "But the Desert Crossing war game in 1999 suggests we would have ended up with a failed state even with 400,000 troops on the ground."

There are currently about 144,000 U.S. troops in Iraq, down from a peak of about 160,000 in January. Read more…