Many Americans fighting in Iraq and Syria are Special Operations Forces, which fall into a gray area between regular troops and clandestine operatives. Under the 1973 War Powers Act, which is still in force, military deployments require congressional authorization after 90 days. CIA operations do not.
A few days ago, the United States also deployed 100 Army Rangers to Manbij, a small Syrian city about 50 miles northeast of Aleppo and 25 miles south of the Turkish border. It’s more overt than most Special Operations missions, because their goal is to get between Syria (and their Russian backers), Turkey, and the American-supported force attacking Raqqa further east.
For better or worse, the United States has given the executive complete discretion about deploying Special Operations Forces. But the Marines are a branch of the regular military.
Additionally, while some Marines already operate in Iraq, the Iraqi government gave them permission. The Syrian government has not. Embattled Syrian President Bashar al-Assad, flush from his victory over non-ISIS rebels in Aleppo, called the American forces “invaders.”