THEY HAVE NOTHING TO LOSE

Iran's military plans for invasion by U.S.
Iran, apparently anticipating an American invasion, has quietly been restructuring its military and testing a new military doctrine that calls for a decentralized, Iraqi-style guerrilla campaign against an invading force.

Ahmadinejad probably sees it as a win - win.
(a) He does not care that Iranians would lose their lives.
(b) It would rally the citizens, creating nationalism, even many that are pro-West would rally to the cause against "Satan".
(c) It would give the United States another black eye in the view of the majority of the nations around the world.
(d) It would cause more unrest back in the U.S.
(e) It would increase his stature in the Arab world.

He probably see no downside at all.