Kurds in northern Iraq are casting their ballots in a referendum that moves the region closer than ever before toward full independence. But outrage in Turkey over the measure cast a pall over the historic vote.

Turkey, like other neighbors, has made it clear it will no longer be business as usual with the Kurdistan Regional Government (KRG), which called the plebiscite to crown a quarter-century of autonomy for what is considered the world’s biggest ethnic group without a state.

Just hours after polls opened, Turkish President Recep Tayyip Erdogan called the vote “illegal and illegitimate” and threatened to close the border and cut oil flows. Chillingly, he left open the possibility of military action.

“This referendum is unacceptable. We stood by them during their most difficult times,” Erdogan said in a televised speech, referring to the close economic and political ties Ankara and Erbil have cultivated over the last decade. He then alluded to a potential Turkish military operation in Syria, adding, “We will not shy away from doing the same in Iraq.”