As a result of the secret 1916 Anglo-French Sykes–Picot Agreement Britain gained control of the Ottoman Vilayets of Mosul, Baghdad and Basra, which it organised into the mandate of Iraq in 1920.

By the 1920 Treaty of Sèvres Anatolian Turkey was to be partitioned, with the areas north of the Mosul Vilayet to be included within an autonomous or independent Kurdish state.

[5] As a provisional measure, the former northern border the Mosul Vilayet was to serve as a frontier between British-controlled Iraq and Turkey, with a more precise delimitation to be agreed upon later.

[3] British and Turkish officials met in 1924 but were unable to determine a mutually satisfactory border, and the matter was referred to the League of Nations.

[3][6] After further deliberations, the League formally recommend in July 1925 that the Brussels line be utilised, a view endorsed by the Permanent Court of International Justice at the Hague in November 1925.