Russia Ukraine war, page-1607

"Which USA Military units invaded Syria and when?"

First USA military units were al-qaeda/ISIS and islamic militants local and imported via Turkey, fresh from murdering Gaddafi and destroying Libya.
And also Kurdish militants.
https://en.wikipedia.org/wiki/List_of_United_States_attacks_on_Syria_during_the_Syrian_civil_war

When they failed to overthrow the Syrian Gov and murder Assad due to Russian intervention the US directly occupied Syrian oilfields.

The US has ramped up its military presence in Syria after a number of skirmishes with Russian forces intensified tensions in the country.

US officials said six Bradley Fighting Vehicles and about 100 troops were part of the deployment to north-east Syria.

Incidents between US and Russian forces that patrol that part of the country have escalated this year. https://www.bbc.com/news/world-middle-east-54215915

It's hard to know exactly when the first official US military units occupied north Syria but it was the US failure in Syria, due to Russia, which led the US to conclude they must destroy Russia ASAP.