he is responsible for ISIS related rebels moving against Assad, who they have been fighting before 2014 when Obama opted not to strike. Not to mention, the Russians were already there in 2014.
The red line crossed was that Assad used helicopters to gas people in an area held by these rebels. The Russians having bases there made the situation incredibly difficult for the US. The US was able to push Lavrov (Russian foreign minister) to get Syria to agree to allowing the US to have their chemical weapon stock pile which was destroyed. That was a more effective resolution to Assad using chemical weapons than a tactical strike ... similar to the destroying airport runways by Trump. The runways were fixed within a few weeks
What Russia did there was to attempt to blame the rebels for that attack and others, even though they had no planes and the attacks were from planes or helicopters.
In the years when we were fighting ISIS, the number of factions made for a complicated, perhaps unsolvable puzzle. Assad was, by 2014, a brutal tyrant as his father had been. The rebels against him ranged from pro democracy people, especially around 2012, and various jihadists. Even factions that were not jihadists were aligned with the jihadists who they saw as the most capable of those against Assad.
In parallel, in Iraq, ISIS converted many Sunni people who had been supporters of Saddam Hussein. By the time, Obama created a coalition to fight ISIS, ISIS controlled a large area of Syria and Iraq. The coalition deconflicted efforts against ISIS by Iran, Russia and Syria. Among the Iraqi forces working with the coalition, some of the strongest of whom were the Iraqi Kurds.
Turkey, a member of NATO, labelled various Turkish Kurd groups terrorists and they complicated working with Iraqi and Syrian Kurds. When Trump came to power he abandoned the Kurds.
The problem, then and now, is that both the Assad government and the jihadists are bad.
= new reply since forum marked as read