The U.S. troops occupy a part of Syria where they have Al-Tanf base.
Looking for reasons why is that, I read from the same Wikipedia source:
U.S. National Security Advisor John Bolton said in early 2019 that U.S. operations in the al-Tanf area would continue as a part of the U.S. effort to counter "Iranian influence" in Syria
Ok, but is that a valid reason, and does it provide legal justification under international law?