It is fair to say that the Persian Empire was in utter chaos during the years following the constitutional revolution of 1906 and Reza Shah's ascendence to power in 1920's.

During this period the central government -governed mostly by the Majles (the parliament) with powerless Ahmad Shah, a teenager, sitting on the throne- had effective control over Tehran and the vicinity only. Southeast Persia was under de facto British occupation while north of the country was under the influence of Russia. Meanwhile several regions were under control of various rebel groups and/or self declared independent regimes: e.g. Jangali rebels in the Caspian, Azadi Republic around Tabriz, Colonel Pasyan declaring an independent government in Mashad etc...

On top of this, the weakness of central government enabled all tribal confederations to act as independent political forces; for example the Bakhtiyaris and the Qashqa'i effectively ruled their territories -south of the country- and cut deals with foreign powers. They had formidable number of armed men and held immense power over the country. They became crucial political actors; when Mohammad Ali Shah dissolved the parliament in 1909, the Baktiyari cavalry forces rode from the south to Tehran and forced the shah to abdicate and place his 11 year old son -Ahmad Shah- on the throne.

During the War Ottoman forces invaded Tabriz and remained there for a couple of years as a part of their Caucasus Campaign, they withdrew towards the end but were replaced by Azadi Rebels declaring their own republic. Britain controlled south/southwest and in 1917 used Persia as a springboard for a failed military intervention against the communist revolutionaries in Russia, in response the Red Army actually invaded north of the country and forced Ahmed Shah to agree to a set of humiliating concessions.

In short, Persian Empire was a unified political entity only in name during World War 1. In reality foreign powers, tribal confederacies, regional rebels, and provincial elites controlled various parts of the country at various times while the central government was stuck in Tehran and effectively powerless. Reza Khan rose to power under such circumstances and through harsh policies, military victories, and British support managed to reestablish Tehran's control over rest of the country and while doing so got himself crowned as the first Shah of Pahlavi Dynasty.