Several issues w/ Mahdi doctrine.
1) Ahmadinejad belongs to a right-wing faction of Islamic regime. He & his faction (some of them extreme far right) are/were members of the “Hojjatieh” society, which has always had a staunch belief in the return of the 12th imam (Mahdi or Mehdi).
2) Hojjatieh society (extremely messianic in nature) is a relatively small part of the overall Islamic regime in Iran, and the Regime’s top/key players.
3) Hojjatieh was even too extremist for Khomeini (original founder of the Islamic Republic regime), he even banned them.
4) Present supreme leader (Khamenei), not to be mistaken for “Khomeini”, is not a supporter of Hojjatieh & the apocalyptic vision Ahmadinejad subscribes to. At least not in those exact terms.
5) Although most Shi’ites in Iran, even Sunnis believe in the return of an Islamic figure before the end of times (similar to biblical accounts of “Armageddon”), they believe “the global War” leading to it will be started by the West.
6) After the return of Mahdi, they believe we (the world) will convert to Islam, and will be rid of corruption, decadence, etc.. - so, they think by forcing people to convert to Islam now & the introduction of Islamic gov’ts throughout the region & in fact the world, they’re paving the way for the return of Mahdi.
But, they also believe “the final battle” will be started by the West (or non-believers in Islam). The annihilation of Israel, and generally the West (also known by them as Great Satan (US) & Little Satan (Israel) is in this context.