They are part of the patchwork of eastern Christian communities who trace their origins back to ancient Mesopotamia, modern-day Iraq, well before Christianity and later Islam took root.
In broad terms, Assyrians are Christians who remained faithful to the Nestorian doctrine, condemned by the Council of Ephesus in 431 AD.
They affirm that Jesus had two separate personalities --one human and one divine -- while a majority of Christians hold that he has a single personality that is both human and divine.
Tradition says that it was two of Jesus's apostles, Thomas and Thaddeus, who took the new faith to Mesopotamia in the first century. In 37 AD, Thaddeus was the first bishop of Seleucie-Ctesiphon, the capital of Parthia that is near to Baghdad, while Thomas left for India.
What is certain is that from the end of the first century, missionaries from Palestine or Antioch, in modern-day Turkey, settled to the west of the Parthian empire close to Babylon's Jewish and High Mesopotamia's Aramaic communities.
The latter, speaking Aramaic, claimed descent from the Assyrian empire and gave their religious community the same name.
Assyrian Christianity later split between those who aligned themselves with the Vatican and those who remained faithful to the original beliefs.
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Those who chose to follow Rome became known as Chaldean Christians, a community that is mainly found in Iraq.
Some 30,000 Assyrians lived in Syria before the start of the civil war there in 2011. Two thirds of them in Hasakeh province, in the far northeast of the country, where the recent abductions occurred.
They are mainly based between the city of Hasakeh and Ras al-Ain in 35 villages around Tal Tamr.
Their settlement dates from 1933, when the community in Iraq was massacred following an uprising.
France settled the surviving community members in agrarian communities, with some then moving on to Hasakeh and Qamishli cities in the province, or to Aleppo and Damascus.
According to the French Roman Catholic newspaper La Croix, the Assyrian church counts between 250,000 and 400,000 followers, mainly in the United States and India.
The current patriarch of Seleucie-Ctesiphon, Mar Dinkha IV, in the 1980s established the seat of his church in Chicago.
In Syria, local Assyrian communities have formed small defence forces that are fighting alongside Kurdish militants in some places.