Destroying fortifications on the Syrian border [1], providing intel to Turkey.
There's an official denial of a "green light", but it's pretty clear that that's exactly what it was -- a surprise withdrawal and support ahead of Turkey's offensive. U.S. hardware, support, and consent.
It might as well be U.S. troops slaughtering them now, but the U.S. press seems to be treating it as something that the U.S. was not deeply involved in, or something deeply shameful for the U.S.
[1] https://twitter.com/sojtfoir/status/1175869891305451522
[2] https://www.nytimes.com/2019/10/09/world/middleeast/turkey-a... ("Because of an American counterterrorism partnership with Turkey, Turkish aircraft were given access to a suite of American battlefield intelligence in northeast Syria.")