ISIS Begins Imposing Jizya on Christians in Mosul

Fight such of those to whom the Scriptures were given as believe in neither God nor the Last Day, who do not forbid what God and His apostle have forbidden, and do not embrace the true Faith, until they pay the tribute by hand with abject submission. (Koran 9:29)

• From ”The Dhimmi” by Bat Ye’or:
  The dhimmi [non-believer] had to pay a poll tax, the jizya (Koran 9:29), which was assessed at three rates in accordance with the economic condition of each individual male above puberty. According to some jurists, this poll tax was to be paid by each person individually at a humiliating public ceremony in which the dhimmi, while paying it, was struck either on the head or on the nape of the neck. However, Abu Yusuf Ya’qub (d. 798) recommended clemency and justice in the collection of the jizya. Women, paupers, minors, the chronically sick and the crippled were theoretically exempted from this tax. When oppressive taxation provoked a wave of conversions to Islam, the administration imposed a collective sum on each dhimmi community which their notables apportioned between them, irrespective of their numbers. Possession of the receipt for the jizya – originally a  piece of parchment worn around the neck or a seal worn on the wrist or on the chest – enabled the dhimmi to move from place to place. A dhimmi
  traveling without this receipt could be put in jail. The seal of the jizya, characteristic of the dhimmi, was soon regarded as a mark of dishonor. (page 53-54)

One reason why the population of the Middle East is largely Islamic is due to the jizya. To be a dhimmi in Islamic society was to be lower than a second-class citizen. As a result, dhimmis (non-believers) were subjugated to harsh social segregation and political violence. Many non-believers converted to Islam to escape such treatment which led to over the years a decline in non-Muslim communities. The Christian Copts for example were once the majority in Egypt . . . now they are a persecuted minority with no rights or privileges. Though Muslims no longer enforce the jizya on non-believers anymore the social status of dhimmitude is still in effect. (more here)

Throughout history it was the  jizya the financed Islamic military campaigns of conquest.

Robert Spencer explains jizya in full here and here.