There is so much misreporting on this, even by people like Robert Spencer, Bernard Lewis, etc.
The Mahdi is the 12th Imam only for the Shi'ites; for the Sunnis, Sufis, etc. the Mahdi will be some Muslim almost as great and pure as Mohammed himself.
He comes with this twofold purpose: to kill the Dajjal, and by that act to establish a world-wide Islamic state governed by shari'a law.
It is not amorphous "chaos" that causes the Mahdi to appear -- it is the presence on earth of the Dajjal ("the Deceiver") -- the greatest evil to beset the planet since the serpent in Eden.
Here's the key: For the past several decades, many Muslim preachers have identified Western culture, the Western nations, and the Western people -- collectively -- as "the Dajjal." Ergo, if we exist on Earth, the Mahdi will soon appear to kill us all and to wipe the planet clean of us.
The Hadith foretell that the Dajjal will take over / have influence on nearly the whole planet (all except for Mecca and Medina) in the Last Days. Muslims are interpretting that as the pervasive influence of Western multinational corporations, Western governments, Western culture, Western technology, etc.
The Hadith further prophesy that the Mahdi and his small vanguard of "true" Muslims will overthrow the Dajjal and kill all of his followers. That explains the unquestioning conviction the Muslims have that they will triumph.
Neither the Qur'an nor the Hadith gives a date, but there is a widespread belief that Judgment Day will arrive in the Muslim year 1500 AH, or 2076 AD.
Ahmadinejad believes that he can best "pave the way" for the return of the Mahdi by creating a "Mahdaviat" society -- that is, by creating a society based on strict interpretation of shari'a law and the rejection of all "Dajjalic" Western influences. Thus, when the Mahdi arrives, he will be able to just slip into the driver's seat to lead the jihad against the Dajjal.
See my FR homepage for more information.
RE: #12 above
Thanks for that insightful exposition.
The Mahdi must be named Mohammed and be a direct descendant. Doesn't that leave out Osama -- or does he have an unused middle name? Dajjal, tell me your fascination with the Mahdi - do you have new insights?