With the 3rd clause of the Treaty of Lausanne signed on July 24, the frontier between Turkey and Iraq was regulated. This clause, where Mosul was not mentioned, stipulated that Turkey and Britain would continue negotiations for another 9 months to determine the frontier and that the Mosul issue would be taken to the Council of the League of Nations unless an agreement was reached. However, the clause had been unclear about the methods of the League of Nations or just what the decision would be about. As mentioned before, taking the issue to the League of Nations would in any case be in favor of the British interests. Turkey would clearly have no say in an organization that it wasn’t even a member of. The British deep state, which had maintained a strong grip over the League of Nations back then, pushed with all its might to make illegitimate claims over Mosul.
When the issue was eventually referred to the League of Nations, Turkey asked that the League establish ‘the will of the people of Mosul’, but Britain asked for the establishment of the ‘frontier between Turkey and Iraq’. The fact that the respective clause in the Treaty of Lausanne didn’t make reference to Mosul, but only mentioned the ‘frontier’, gave the British argument the upper hand (SOURCE)