Khomeinist theologians believe that the destruction of Israel is a precondition for the Hidden Imam to reappear. I.e. as long as Israel exists won’t the Hidden Imam reappear and so is Israel according to this particular theology an obstacle to the emergence of the messianic age. This is why theocratic Iran is so obsessed with in the words of its leaders “annihilating Israel”.
Yet what is the best strategy for destroying Israel from the perspective of Iran’s strategists? Iran is pursuing nuclear weapons for strategic reasons, yet knows that Israel and America can repeatedly neutralize Iran’s nuclear weapons program. The only way Iran can conceivably prevent this is by destroying Israel and mete out such a blow with biological weapons against the American homeland that the US will never again (according to this thinking) dare to neutralize the Iranian nuclear weapons program. The alternative would be for Tehran to permanently disband its nuclear weapons program.
Another alternative for Iran would be to absorb the blow of the neutralization of its nuclear weapons program without ordering military action and take vengeance in a planned WWIII whereby both Iran and Russia would simultaneously launch westward invasions of the Arab world and Europe respectively and thus force the US to fight on two fronts simultaneously which would be militarily advantageous for both Tehran and Moscow.
The counterargument is that it would be better to destroy Israel (Iranian strategists imagine that the Iranian military is capable of that) a few years before conquering the Arab world so as to prevent the development of the nascent Israel-centered MENA defense alliance.
However, everything points to Iran preparing two separate wars, the first against Israel in Syria and the second against the Arab world.