Syria has long borders with other Arab countries and these borders often cut across territories regarding as one entity by tribal groups. So people hopping to and fro from Iraq, Lebanon and Jordan are impossible to police.
Many of these people are from groups and / or religious sects that hold extreme views, such as the Druze, Alawi and Shia Islamic groups from Iraq in particular. The Shia have long had a cult of personal martyrdom, giving rise to the Nizari 'assassins' and the present numerous extremist groups.
Most Syrian Muslims are more moderate traditionalists - 'Sunni' Muslims, and pockets of Shia follwers were interspersed across the oases of the interior of Syria until the recent conflicts started.
Syria also has (or had) many Christian sects and Damascus for example had both Christian and Jewish residents as well as Muslims. Many of these were ethnically Arab.
Ethnically most Syrians are Arab but there are (or were) areas that held predominantly Kurd and other minoritis such as Turkmen and Circassian.
Like anywhere these many different groups have rubbed along more or less but since the 1960s the number of extremists getting into Syria via Lebanon has increased.
Assad's family are Alawi Muslims, a sect that was shunned by most other Muslims in the area and which in response has become secretive. Assad appears to have little regard for any of the people of Syria or preserving its diversity.
I suppose it's what it might have been like under the Normans in England, but with high-powered technology.