Until the Arab invasion and subsequent Muslim conquest, in the mid-7th century Persia (modern-day Iran) was a politically independent state, spanning from Mesopotamia to the Indus River and dominated by a Zoroastrian majority. Zoroastrianism was the official state religion of four pre-Islamic Persian empires, the last being the Sassanian empire that passed a decree solidifying this in 224 CE. The Arab invasion abruptly brought to an end the religious domination of Zoroastrianism in Persia and instituted Islam as the official religion of the state.