On the eve of Monday’s referendum on the future of Iraqi Kurdistan, Iranian armed forces conducted a significant military drill on the border between Iran and Iraq as Tehran warned the Kurds not to move forward with the plebiscite. Meanwhile, Iran’s Supreme National Security Council—the entity in charge of crafting and communicating the country’s security strategy—announced that it would halt all flights to and from the Kurdistan region’s major airports, Sulaymaniyah and Erbil. According to Tehran, the council took that step after Baghdad requested it to do so. All this activity is hardly surprising; in a region plagued by conflict and terrorism, the Kurdish referendum may just be Iran’s greatest challenge yet. And now that the Kurds have voted for independence despite regional and international opposition, they’ll be starting what will likely become a long, multilayered negotiating process to achieve secession.

Iran has a longstanding relationship with Iraqi Kurds, and it’s the only country in the region with a substantial Kurdish population to have consistently preserved decent relations with them. Iranian support for the Kurds has received more attention since 2014, when Iran backed certain Kurdish efforts against the Islamic State (ISIS), but the ties go back decades.

Even before the 1979 Islamic Revolution, the Shah provided support to the Kurds in Iraq. In those years, Iran, joined by Israel and the United States, supported a Kurdish insurgency against the al-Bakr government in Iraq. At the time, Tehran’s backing of the Kurds served two main purposes. First, the Shah shared the United States’ fears about the spread of communism in the Middle East, and the Soviet Union backed the government in Baghdad. Second, the Shah wanted to check Iraq, whose claims over parts of the river bordering the two countries—the Shatt al-Arab, as it’s known to the Arabs, and Arvand Rud, as it’s known to Iranians—had caused longstanding tensions between the two capitals.

The dispute ultimately led to the fight: this one against ISIS, in which the Kurdish Peshmerga fought side by side with the Islamic Revolutionary Guard Corps’ elite Quds Force and Iranian-backed Iraqi Shia militias.

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