For the posters on here who are ignorant of history. You can simply look up Wikipedia and find the details (or linked articles).
en.wikipedia.org
From Wikipedia:
From the 7th century AD onwards, Mesopotamia saw a steady influx of Arabs,
Kurds and other
Iranian peoples,
[113] and later
Turkic peoples. Assyrians were increasingly marginalized, persecuted and gradually became a minority in their homeland. Conversion to Islam was a result of heavy taxation, which also resulted in decreased revenue from their rulers. As a result, the new converts migrated to Muslim garrison towns nearby.
Basically, they got supplanted in their own lands very similar to how Americans supplanted Native Americans.
Further details on how they were persecuted and even slowly exterminated (notably by Ottomans):
Indigenous Assyrians became second-class citizens (
dhimmi) in a greater Arab Islamic state. Those who resisted
Arabization and conversion to Islam were subject to severe religious, ethnic, and cultural discrimination and had certain restrictions imposed upon them.
[109] Assyrians were excluded from specific duties and occupations reserved for Muslims. They did not enjoy the same political rights as Muslims, and their word was not equal to that of a Muslim in legal and civil matters. As Christians, they were subject to payment of a special tax, the
jizya.
[110]
They were banned from spreading their religion further or building new churches in Muslim-ruled lands, but were expected to adhere to the same laws of property, contract, and obligation as the Muslim Arabs.
[110] They could not seek the conversion of a Muslim, a non-Muslim man could not marry a Muslim woman, and the child of such a marriage would be considered a Muslim. They could not own an enslaved Muslim and had to wear different clothing from Muslims to be distinguishable. In addition to the jizya tax, they were required to pay the
kharaj tax on their land, which was heavier than the jizya. However, they were protected, given religious freedom, and to govern themselves according to their own laws.
[111]
As non-Islamic
proselytising was punishable by death under
Sharia, the Assyrians were forced into preaching in
Transoxiana,
Central Asia,
India,
Mongolia and
China where they established numerous churches. The
Church of the East was considered to be one of the major Christian powerhouses in the world, alongside
Latin Christianity in Europe and the
Byzantine Empire (
Greek Orthodoxy).
[112]