Persia in 1914-1918 was a lot like Mexico: it was on the maps but not so much in reality. The central government did not have control beyond the capital and the countryside was either in rebellion, in anarchy or occupied by Russian and British troops guarding their own spheres of influence.
So you have to use an earlier POD to strengh Persia (the latest would be something realted to the constitutional revolution of 1905). That would considerably increase the Allied troops in the region, but the Persians would still be a lot weaker than the Ottomans (both in weapons and leadership). And I doubt Russia and Britain would refuse to their ambitions over the area in favor of Persia, by the way.