Five days ago, the 3rd Ranger Battalion was spotted heading towards the city of Manbij. The U.S. rangers were presumably deployed to join the SDF, a coalition of Kurdish militias operating in Northern Syria, in their siege against the ISIS occupied city of Raqqa. At around the same time, the Pentagon deployed 400 marines of the 11th Marine Expeditionary Unit to also join the Kurdish forces fighting to push ISIS out of Raqqa. 6,000 U.S. troops are currently operating between Iraq and Syria. The Pentagon plans to deploy an additional 2,500 troops to Kuwait as backup for the forces in Iraq and Syria.
The invasion was started by the Obama administration in December of 2015. By the end of his last term, Obama had deployed a total of 500
Special forces into Syria to assist the SDF. The recent deployments are thus not a change in policy, but simply an expansion of an Obama era policy. Obama also sent special forces to three other countries (i.e. Yemen, Iraq, and Libya).
Article I Section 8 Clause 11 of the Constitution gives Congress the sole power to declare war; being the commander and chief does not give the executive office the prerogative to start wars on a whim because the president does not have the power of the purse (for obvious reasons). Furthermore, the War Powers Resolution statute requires the president to notify congress within 48 hours of deploying troops and to remove all troops within 60 days if the commitment does not receive formal congressional approval. In this capacity, both president Obama and Trump have violated the law.