The term originated in the days before Muhammad to refer to a politically-active class of slaves and freedmen, but it gained prominence during the Umayyad Caliphate, as many non-Arab subjects converted to Islam. The influx of non-Arab converts to Islam created a new difficulty in incorporating them into tribal Arab society. The solution appeared to be the creation of a contract, a wala', through which the non-Arab Muslims acquired an Arab patron (mawla). They continued to pay a similar tax that was required from the people of the book and were generally excluded from government and the military until the end of the Umayyad Caliphate. In Khorasan and Persia, the Arabs held most of the higher positions in the armed forces and in the upper echelons of government.