I don't want to hear that Trump doesn't know, unless I'm to believe that no members of the National Security Council have informed him, and that the facts about what the U.S. military is up to in Syria have been withheld from the presidential briefings.
The American people repeatedly suffered blowback from U.S. support for terrorist groups and tyrants during the Cold War. Yet actions by successive American presidential administrations since the end of the Cold War showed that nothing had been learned, nothing had changed. I'd hoped, briefly, that Trump would be able break the cycle. If he tried, the nightmare in Syria is evidence that he failed.
There's really nothing more to be said. Explanations, arguments, trotting out of evidence -- all that is the attempt to persuade people who are sincerely making a mistake. The American defense establishment is not making a mistake. Its leaders know exactly what they're doing. Assad remaining in power, and threats from Iran and Russia -- all that is phony rationalizations for aiding not only al Qaeda but also other terrorist organizations, including Islamic State, operating in Syria. This aid started during the Obama Administration, but the Trump Administration doubled down.
To ask why is useless. Evil does not need a reason.