Iran is much too much of a mouthful to attempt to invade. All we'd do is to smash up a few dozen more heritage sites, and stir up a hornet's nest amid the Islamic World that would cost us a fortune in security measures.
What has Ahmadinejad actually done? He has a robust Opposition, who might have won the election fair and square (but we are not certain of that), he has issued curses and dark threats against the Israelis (with some justification, but I do not see any serious attempts at repeating the final solution Hitler tried), and his engineers have flaunted the nuclear proliferation treaty (arguing that Israel, Pakistan and North Korea - none of whom are respectable democracies in his opinion - have them, why not Iran?).
We have every justification to be deeply suspicious about Iran, but the correct military response here is an effiicient intelligence operation (so we know what is really going on), and a serious push to win hearts and minds among the Iranians, many of whom have strong sympathies already with the West, so that Ahmadinejad and the Ayatollahs are isolated. More importantly their efforts are concentrated on internal oppression, rather than military adventures overseas.
I cannot see France (which gave sanctuary to the Ayatollahs when they were in exile during the Shah's regime) co-operating on an invasion. Nor can I see the USA having much of an appetite for more war - they still haven't yet sorted out Iraq and Afghanistan, which are smaller countries.
I therefore cannot see the Lib Dems have much to lose by opposing any invasion of Iran.