To ensure his victory, Putin will do what he can to discourage the U.S. from acting more decisively. That’s why we should take seriously the threat that Russia will supply Assad with S-300 air defense missiles. Putin’s foreign minister, Sergei Lavrov, has said Russia hasn’t agreed to any new contracts for defense assistance to Syria, but old contracts will be fulfilled.

If those old contracts include the delivery of S-300s, it will become much more dangerous for the U.S. and its allies to impose a no-fly zone in Syria. Obama hasn’t seemed very eager to undertake that mission. Nevertheless, I would think he would want to preserve the option in the event that he belatedly recognizes how mistaken his forbearance has proved to be.

Red lines aren’t what they used to be since Syria crossed Obama’s most recent one about using chemical weapons, and the president’s response to date is to have Secretary of State John Kerry get Lavrov’s agreement to an international conference on Syria, the time and place for which is yet to be determined — if it ever happens at all. But now would be a good time to try drawing another one, and let Putin know that if he sends S-300s to Syria, the U.S. will accept his challenge that Russia is again a hostile adversary and we’ll resume treating it accordingly.

The world could benefit right now from a little more of the cool, analytical Obama, who assesses the efficacy of policies by their results, and a little less of the starry-eyed dreamer, who pines for the relationship that might have been.