The Mosul government witnessed a diplomatic crisis that broke out between the Kingdom of Iraq and the Republic of Turkey after World War I over the fate of the Mosul Vilayet. This war resulted in important results on the international level, as Britain and the United States of America paid attention to the outcomes of the Peace Conference in Paris, while the countries that were on the scene of the Allied battles continued to groan from the burden of problems, especially external ones. After losing the Ottoman Empire, the Ottomans signed the Treaty of Mudros with the Allies on October 18, 1918. The sixteenth resolution of the treaty stipulated the surrender of the Ottoman army in the countries of the Levant, Yemen, and the Hijaz. However, the treaty did not define the southern borders of the Ottoman Empire, and the Istanbul government demanded Mosul as part of its territory. In addition, the British army did not enter Mosul until November 15, 1918, i.e., 16 days after the treaty, and this is considered a violation of international law. By the British mandate over Iraq, the British represented Iraq on the international stage, and in this research, we will learn about the reasons that prompted Britain to defend the Mosul Vilayet. The research was divided into an introduction, three chapters, and a conclusion. The first chapter was divided into three sections: the first section is the geographical location of Mosul, the second section is the economic importance, and the third section is the historical importance. The second chapter is the Mosul Governorate, considering international treaties and agreements. It was divided into two sections: the first section was the declaration of the armistice and the entry of British forces, and the second section was the Mosul Governorate, considering international agreements and treaties. The third chapter is the problem in the League of Nations and the final settlement. It was divided into two sections: the first section is the presentation of the problem in the League of Nations, and the second section is the UN investigation committee