With some 50,000 troops, chiefly from India, the British launched their campaign into Ottoman Mesopotamia on 13 December 1916. Facing around 25,000 Turkish troops they made steady progress until, on the 8 March they reached the Diyala River, south of Baghdad.
The next day the first British assault on Ottoman defensive positions occurred and the following day saw tactical moves splitting & partially defeating the Turkish forces, the remaining troops retreating north.
With Baghdad undefended, the Ottoman authorities ordered an evacuation at 8 p.m. on 10 March and the British entered the city without a fight on 11 March 1917. The aftermath left over one third of the original Ottoman force as prisoners of the British.
It took some time for the British to produce stamps for Occupied Bagdad, as they were busy moving toward Mosul, and were no doubt distracted by arguments between London and the British Indian Government over who should exercise direct control (obviously a time where all official proclamations, including stamps, had to be carefully worded), but on the 1 September a series of surcharges reading "IN BRITISH / BAGHDAD / OCCUPATION" were applied to captured Turkish stamps.
These are Iraq's first stamps, for earlier material Iraqi philatelists must hunt postal markings on covers or as applied to the stamps of chiefly India or Turkey. There are 25 surcharged stamps, several have errors, but the rarest of all the basic stamps is the 2a surcharge applied to the Turkish 1pi bright blue depicting the Mosque of Sultan Ahmed with a small five- pointed star in red overprint.