Was the gassing of civilians the Syrian people speaking as Rex Tillerson proposed when he said that the “longer-term status of President Assad will be decided by the Syrian people” ?
Trump tweeted repeatedly that then president Obama should stay out of Syria. After Assad had crossed the line, Obama asked for congressional approval of major military action that would have crossed the line for the Russians. And the Republican controlled congress did nothing. 
Trump, who famously said, "I alone can fix this" and "Nobody knew healthcare could be so complicated" now has a change of heart about Assad (but he's willing to still blame Obama). 
What led Trump to trust Assad? 
Trump says he will not disclose and retaliatory actions. I doubt he will engage in any military action. With the Russians controlling things as they have with Syria for a long time, there is little that can actually be done without sacrificing our own troops.