Donald Trump has said that we need to focus on fighting ISIS, not Russia.
Dropping a tactical nuke to show Russia who’s boss and to send a message on how tough we are is childish and would only serve to allow Al Qaeda to take over Syria. The largest concentration of Al Qaeda militants in the world are basically in charge of the rebels that we, for some insane reason, are supporting. Most likely to show Russia how tough we are.
We had our way in Iraq and Libya with no Russian interference, so why not give the Russians get their turn in Syria without any interference from us? We have been pumping in about $1B worth of weapons per year to rebels opposing them and providing logistic support and crippling sanctions against the Syrians.