When WWI broke out, the Ottomans didn’t have a significant military presence in Iraq. On August 2, 1914, general mobilization was declared throughout the entire empire and in the days following, the Turkish army was reconstructed on the Iraq front. However, it didn’t look very likely for this army to be able to fight off the regular armies of Europe. It was very difficult to replenish uniforms and equipment or transfer weapons to the area (SOURCE)
Moreover, the number of troops in Iraq was very low. Ostensibly due to the Italo-Turkish war, the Balkan riots and the Balkan wars, the Ottoman army had to shift its focus away from Iraq. However, according to Ahmed İzzet Pasha, one of the Ottoman grand viziers, the real reason was different. The Ottoman Empire never contemplated the prospect that British could launch an attack in the region. In his memoirs, he explains the dilemma:
Even the kids know that the British have had plans for Iraq since a long time ago. Owing to the impressive cultural legacy and history of civilization of Iraq and Mesopotamia, and its reputation that, with good management and utilization, it could rival the fertility of Nile, Punjabi, Sindh and Ganges basins, these places proved their worth for their owners but also stoked the hunger of big occupying states. The tombs and families of Imam al-A’zam, extremely sacred to Muslims and particularly dear to Shia and very sacred to Sunnis, and the tomb of Abdul-Qadir Gilani, very beloved to Indian Muslims are located in Iraq. Therefore, it could be easily seen that Britain, with already a sizable Muslim subject population and with hopes of being defender of Hejaz, would benefit greatly, in terms of its Islam policy, from capturing this region. It was only natural that the British would set its eyes on Iraq, also for the purpose of preventing this strategically important place from being captured by a strong enemy that could pose a future threat against India. Separating Iraq region from local forces is the same as provoking and inviting the British government to invade this property of ours. Therefore, it is a great mistake that more troops are not dispatched to these regions, before the ultimate need arises (SOURCE)
Although Ahmed İzzet Pasha clearly pointed to the severity of the situation in the region and the sinister plans of the British deep state, sufficient troops weren’t stationed at Mosul. Needless to say, the fact that the Ottoman Empire was fresh out of the Balkan Wars at the time played a great role in this.