The first signs of trouble for the Assyrians started in the 13th century...ancient Assyria

The first signs of trouble for the Assyrians started in the 13th century 

The first signs of trouble for the Assyrians started in the 13th century When the Mongols first invaded the Near East after the fall of Baghdad in 1258 to Hulagu Khan. Assyrians at first did very well under Mongol rule, as the Shamanist Mongols were sympathetic to them, with Assyrian priests having traveled to Mongolia centuries before. The Mongols, in fact, spent most of their time oppressing Muslims and Jews, outlawing the practice of circumcision and halal butchery, as they found them repulsive and violent. Therefore, as one of the only groups in the region looked at in a good light, Assyrians were given special privileges and powers, with Hülegü even appointing an Assyrian Christian governor to Erbil (Arbela), and allowing the Syriac Orthodox Church to build a church there.

Aramaic language and Syriac Christianity 

Aramaic language and Syriac Christianity in the Middle East and Central Asia until being largely annihilated by Tamerlane in the 14th century
However, the Mongol rulers in the Near East eventually converted to Islam and then sustained persecutions of Christians throughout the entirety of the Ilkhanate began in earnest in 1295 under the rule of Oïrat amir Nauruz, which affected the indigenous Assyrian Christians greatly. During the reign of the Ilkhan Öljeitü, the Assyrian Christian inhabitants of Erbil seized control of the citadel and much of the city in rebellion against the Muslims.

However, in spring 1310, the Mongol Malik

However, in spring 1310, the Mongol Malik (governor) of the region attempted to seize it from them with the help of the Kurds and Arabs but was defeated. After his defeat, he decided to siege the city. The Assyrians held out for three months, but the citadel was at last taken by Ilkhanate troops and Arab, Turkic and Kurdish tribesmen on July 1, 1310. The defenders of the citadel fought to the last man, and many of the Assyrian inhabitants of the lower town were subsequently massacred.

Regardless of these hardships

Regardless of these hardships, the Assyrian people remained numerically dominant in the north of Mesopotamia as late as the 14th century AD, and the city of Assur functioned as their religious and cultural capital. However, in the mid-14th century the Muslim Turk ruler Tamurlane conducted a religiously motivated massacre of the indigenous Assyrian Christians, and worked tirelessly to destroy the vast Assyrian Church structure established throughout the Far East, destroying the entire structure of the church with the exception of the St Thomas Christians of the Malabar Coast in India, whom number 10 million or so in modern times. After Timur's campaign, The Assyrian Cultural and religious capital of Assur was completely destroyed, thousands of Assyrians were massacred, the vast church structure of the Assyrian Church of the East was decimated, and the Assyrian population was from that point on reduced to a small minority living within Muslim dominated lands.

Around 100 years after the massacres by Timur

Around 100 years after the massacres by Timur, A religious schism known as the Schism of 1552 occurred among the Assyrians of northern Mesopotamia, when a large number of Nestorian(followers of the Assyrian Church of the East) Assyrians in Amid elected a rival Patriarch named Shimun VIII Yohannan Sulaqa after becoming dissatisfied with the leadership of the Assyrian Church, At this point based in Alqosh. Due to a need for an ordination by a metropolitan bishop, Sulaqa went into communion with the Roman Catholic Church after at first failing to gain acceptance within the Syriac Orthodox Church. Rome named this new church The Church of Assyria and Mosul and its first leader Patriarch of the East Assyrians in 1553 AD.

Soon after coming back Sulaqa was assassinated by supporters

Soon after coming back Sulaqa was assassinated by supporters of the rival patriarch in Alqosh but was able to form a new church structure and line of succession known as the Shimun Line prior to his death. This group of Assyrians eventually broke off ties with Rome, moved en masse to the Hakkari Mountains, and returned to the Nestorian faith they once adhered to prior to the Schism of 1552, while still operating independently from the original Assyrian Church structure based in Alqosh.

A decade or so before the Shimun line broke off ties with Rome

A decade or so before the Shimun line broke off ties with Rome, another faction within the Assyrian Church entered into communion with Rome known as the Josephite line, and upon the Shimun line leaving, inherited the now vacant Church of Assyria and Mosul, which was renamed the "Chaldean Catholic Church" in 1683. This is now believed to be due to an error by the Catholic Church, but now due to that error, their followers became known as Chaldean Catholics or Chaldo-Assyrians despite having no ethnic, historical, linguistic, cultural or geographic connections whatsoever to the by now the long extinct Chaldean tribe of south-east Mesopotamia.

Later on in the 1830s the original Assyrian Church of the East structure

Later on in the 1830s the original Assyrian Church of the East structure in Alqosh combined with the Chaldean Catholic one, creating the modern Chaldean Catholic Church structure, which is ironic considering that the only remaining ethnic Assyrian Church to practice the Assyrian Church of the East denomination was the first one to split from the Assyrian Church of the East back in 1552. There was also another Nestorian Denomination known as the Ancient Church of the East, which split from the Assyrian Church of the East due to reforms passed under the rule of Shimun XXIII Eshai in the 1960s, but with the election of Gewargis III in 2015 the churches had a reconciliation and reunited.

In addition to the Eastern Rite Churches

In addition to the Eastern Rite Churches, The Syriac Orthodox Church also has a large number of ethnically Assyrian Adherents, who are known as Syriacs. The Syriac Orthodox Church has 5 million adherents across the globe, mostly in India, but is based in Damascus. However, since the 11th century it was based in the Saffron Monastery of Tur Abdin, and prior to that, it was based in Antioch. Like the Nestorian churches, schisms also occurred within the Syriac Orthodox Church. In 1626 Jesuit and Capuchin missionaries began to proselytize among the Syriac Orthodox faithful at Aleppo, forming a larger pro-catholic movement within the Syriac Orthodox Church. So in 1662, when the Syriac Orthodox Patriarchate had fallen vacant, the Catholic party was able to elect one of its own, Andrew Akijan as Patriarch of the Syriac Church. This provoked a split in the community, and after Akijan’s death in 1677 two opposing patriarchs were elected, with one of those becoming the first Patriarch of the Syriac Catholic Church. This line of succession died out quickly, however, but in 1782 with the election of Michael Jarweh as Patriarch the Ignatius line has been the head of the Syriac Catholic Church since then and also has its base in Damascus.

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