Iraq–Türkiye border

The Mosul question

Before 1918 there was no border here. Both sides were part of the Ottoman Empire, and from 1879 the land south of today's line formed the Ottoman vilayet (province) of Mosul.^[1] On the eve of the First World War its people were about 300,000 Kurds, 170,000 Arabs and 40,000 Turks, with smaller numbers of Christians and Yazidis.^[1]

British troops occupied the vilayets of Baghdad and Basra and at least a large part of Mosul during or after the war. In 1920 Britain received the League of Nations mandate for “Mesopotamia including Mosul”.^[3] The Treaty of Sèvres, signed by the defeated Ottoman government on 10 August 1920, put Turkey's frontier along the northern edge of the Mosul vilayet. It was never ratified.^[1]

A resurgent Turkey rejected Sèvres. The Treaty of Lausanne, signed on 24 July 1923, left the Iraq frontier open. Turkey and Britain were to agree on it within nine months, and if they could not, the dispute would go to the Council of the League of Nations.^[1]

“The frontier between Turkey and Iraq shall be laid down in friendly arrangement to be concluded between Turkey and Great Britain within nine months.”^[3]

The League decides

Talks in Constantinople in May and June 1924 failed.^[3] Britain argued that Mosul was already part of Iraq and that only its northern edge was in question. It asked for a line 5 to 25 miles (8 to 40 km) north of the old provincial border, along higher and more defensible mountains.^[1] Turkey wanted the line along the southern border of the vilayet, which would have kept all of Mosul in Turkey, and said a vote there would show a majority for Turkish rule.^[1]

On 29 October 1924 the League Council set a provisional line, the “Brussels Line”, which followed the northern border of the Mosul vilayet almost exactly. A three-member League commission visited the region from November 1924 to March 1925, and in July 1925 recommended that the Brussels Line become the boundary.^[1]

The two sides disagreed about the Council's role. Britain called it an arbitration; Turkey said it could only mediate with both parties' consent.^[3] The Council asked the Permanent Court of International Justice. On 21 November 1925 the Court advised that the Council's decision would be binding and definitive, and that the votes of Britain and Turkey would not count toward the unanimity it required.^[3] In December 1925 the Council ruled against the Turkish claim.^[4]

The 1926 treaty

Britain, Iraq and Turkey signed the Treaty of Angora (Ankara) on 5 June 1926. The border follows the Brussels Line, with one small change in Turkey's favour: south of the villages of Aluman and Ashuta, so that the whole road between them lies in Turkey.^[1] The treaty also set up a standing frontier commission and a frontier zone 75 km wide on each side, where both countries would try to stop raids across the border. The whole line was marked on the ground by September 1927.^[1]

From the Tigris the line follows the Khabur river and its tributary the Hezil for about 36 miles (58 km). North of Zakho it climbs into the mountains, running along ridges and streams and crossing the Great Zab river, until it follows the Hajji Bak stream up to the Iranian border.^[1]

Kurds on both sides

The border runs through land long inhabited by Kurds, who also live across the borders in Turkey and Iran. In 1925 the League Council suggested that the Kurds of Iraq be given autonomy as part of the settlement, but the 1926 treaty did not mention them.^[1] In 1964 the US Geographer noted that the line divided kinship groups, and that Kurdish herders usually crossed it freely on informal passes, except in times of fighting.^[1]

The Kurdistan Workers' Party (PKK), an armed group that fought the Turkish state for four decades, kept bases in the mountains of northern Iraq for many years. Turkish forces launched offensives and airstrikes there and set up bases of their own.^[5] In 2021 Iraq's foreign ministry protested what it called “violations of Iraqi sovereignty”; Turkey said its operations against the PKK respected Iraq's territorial integrity.^[6] The PKK announced in May 2025 that it would disband and give up armed struggle, and some of its fighters burned their weapons in northern Iraq that July.^[5]