Explanation: During the Ottoman Empire, a… curious… system was employed by the central government. They would, as a ‘tax’, seize children from Christian subjects, raise them as Muslims, and train them for palace service or induct them as slave-soldiers into the ranks of the Janissaries - the main force of the Ottoman Sultan.
Devshirme - Wikipedia

Oof…don’t tell the Islamaphobes.
Interestingly enough, it was… extremely dubious by Islamic law, since slaves are only supposed to be taken from peoples one is actively at war with.