The struggle related to the constitutional movement continued until 1911, when Mohammad Ali Shah was defeated and forced to abdicate. On the pretext of restoring order, the Russians occupied Northern Iran in 1911, and maintained a military presence in the region for years to come. During World War I, the British occupied much of Western Iran, and fully withdrew in 1921. The Persian Campaign commenced furthermore during World War I in Northwestern Iran after an Ottoman invasion, as part of the Middle Eastern Theatre of World War I. As a result of Ottoman hostilities across the border, a large amount of the Assyrians of Iran were massacred by the Ottoman armies, notably in and around Urmia. Apart from the rule of Aqa Mohammad Khan, the Qajar rule is characterized as a century of misrule.
Try to answer this question if possible (otherwise reply "unanswerable"): The British occupied Western Iran from World War 1 until finally withdrawing in what year?
1921