For the past two years, U.S. and coalition aircraft have flown in and around one of the world’s more robust air defense systems without the Syrian regime harassing the pilots. We had a few incidents where Russian jets got too close to U.S. aircraft or Syrian anti-aircraft radar lit up U.S. or coalition aircraft, but for the most part, the air war has gone forward unimpeded.

Both Russia and the Syrian regime, though, are still well-positioned to play the spoiler. They can affect the flights of U.S. aircraft in eastern Syria by activating their air defenses and have, in recent months, brought in more advanced air-defense weaponry that has even the Israelis nervous. They’ve also “accidentally” struck U.S.-backed rebel groups fighting the Islamic State.

How will the regime respond? I have no idea. Perhaps, now that they understand force is on the table, they will meekly accept what the United States has just done. Or they can begin to harass coalition pilots or more U.S.-allied rebels. Again, I don’t know. But I do know that America’s coalition partners in Syria and Iraq are all likely much more nervous about what this means for their own forces. Secretary Mattis is going to need to make some calls to soothe some jittery allies.