One likely scenario.
Our warships launch Tomahawks at Syrian targets.
Russian warships closer to Syria launch interceptor missiles to try to shoot them down.
Our warships see these Russian missile launches and think they are being attacked. They launch missiles at the Russian warships.
The Russian warships see the inbound missiles and launch their missiles at the American warships.
World War Three ensues.
Or, Obama abjectly backs down, and as part of the terms, agrees to remove the U.S. Navy from the Med and Indian Ocean, for example.
Sometimes military paradigm shifts happen violently. Ask the Brits what happened to “Fortress Singapore” and their battleships there.
Didn't we knock down an Iranian passenger jet in similar circumstances some years back?