Giving the green light for a Turkish invasion of Syria endangers U.S. Kurdish allies. The Kurds are one of the few reliable long-term partners in the region so the President of the U.S.A. needs to watch out for their security as well as repair the relationship with N.A.T.O. member Turkey that has decided to buy Russian military equipment that is a danger to N.A.T.O. security.
While a Turkish presence in Syria might be easy to allow and will provide a measure of regional security they may be hard to get rid of as the former colonial power (of the Ottoman Empire). It would be helpful if President Trump would explain to the public what is plans are for Kurdish security and for the problem of Syria that the Obama administration started toward civil war and regional insecurity.
It would of course be great if President Trump could in some way get the Kurds and Turks to work together to secure mutual geographic interests. Actually that shouldn’t be too challenging if the Turks are not indignant and meat-headed about it. Maybe a redesign of borders and citizenship issue could be creatively solved at some kind of camp David think tank for Kurds and Turks so they can work together for long-term regional security including Syria.