I'd like to avoid partisanship in this thread. I don't care about trump, the Republicans, or the democrats. I just want to talk about these questions.
If regime change is the new policy with Assad, how do we accomplish that military objective without fighting Iran and Russia?
If we succeed at ousting Assad, who takes over?
Regime change is a political objective, not a military objective. Obama tried over and over to negotiate the ouster of Assad but he was never taken seriously by Russia or anyone else. Now when President Trump calls for negotiations Russia and Iran will take him seriously because they understand that for the first time in eight years all options are on the table. Either we decide we have no interests in the ME, and even Rand Paul is unwilling to say that, or we show the Russian and Iranian aggressors we are serious about defending our interests in the ME. This is not the opening volley of a war, it is the opening statement of a new negotiation to end the war.
Who will replace Assad? No one knows but it is difficult to imagine anyone could do more damage to the Syrian nation or the Syrian people than Assad is doing.