If diplomacy has any chance to work, it must be coupled with a credible military threat.
The minute Mr. Putin decided to put his military forces in Syria, I went to see him. And I said, "Look, here's what I'm doing. I'm not intervening in Syria. But at the same time, if Syria tries to intervene with us, if Iran tries to use Syria to attack us, we'll stop it.
The application of military force, or the prospect of such application, inhibits terrorist violence.