Kurdistan’s planned referendum is a matter of mutual concern to both Iran and Turkey. To Tehran, the referendum — and later the possible independence — introduces three main threats. First, a Kurdish state in Iraq could ignite a Kurdish domino effect in the region, which would enhance the dreams of Syria’s, Turkey’s and maybe Iran’s Kurds. Bitter memories from the days of the 1979 Revolution can’t be forgotten when Iranian Kurdish militants attempted to secede; this was followed by an insurgency that is still active in areas on the country’s northwestern borders.
The second threat is that an independent Kurdish state would mean the division of Iraq and a possible Iraqi Sunni tendency to call for a similar referendum, ending up with at least three small nations in the neighboring country with all the vulnerabilities that might result from such a situation. Iraqi Shiites aren’t fortified against possible internal strife, given the renewed divisions among them with Saudi Arabia’s thriving attempts to attract some factions to its camp.
Another issue to worry about if Iraq gets divided is Syria — borders are going to be loose once again, and there’s no guarantee the de facto government will be ready to cooperate with the Iranians and their allies.
The third and final threat from Tehran’s point of view is that an independent Kurdistan, given the Kurdish autonomous region’s good ties with Israel, will give Israel the chance to spy on Iran and interfere and conspire against it; this will pose a great threat to the Islamic Republic, the Islamic revolution and, thus, to Iran’s national security and stability.
Given the above threats, as well as the Turkish reasons to be worried, Turkey and Iran seem to be on the same page and facing almost similar threats, except for Syria. The existential threat the Kurds pose to Ankara and the threat to Iran’s national security in this regard makes it inevitable for both sides to join forces and cooperate, despite differences, to prevent themselves from facing an imminent threat that might have dire consequences.