The fight is pretty much the US and ISIS against Assad and Russia. Sure Russia is bad, but I think ISIS (and related Muslim extremists) are worse.
Assad, for all his faults, is a secular leader, not a radical Islmaist. Obama has plowed through Egypt, Libya, and Syria toppling secular leaders and trying to get radicals in power. I don't support that agenda.
Russia is in bed with Muslim extremists, openly and likely secretly. In fact, I suspect they had a hand in the 911 attacks. What better way to launch their long-awaited "comeback" following their so-called collapse. Al qaeda #2 was reportedly trained by the Russians. The guy who made that claim, and much more about Russia and terrorism, wound up dead via a poisoning linked by a recent court to Putin himself. They found that he must have known about it beforehand and probably gave the order.