Henry Kissinger reminds us that in international relations, states do not have permanent friends or enemies, only interests. That lesson reverberated Tuesday in St. Petersburg, where Turkish President Recep Tayyip Erdogan let bygones be bygones with his "dear friend, the esteemed Vladimir" in an ironic (and somewhat excessive) display of diplomatic reconciliation. Over the course of only seven months, Turkey and Russia have gone from ranking each other as public enemy No. 1 to catching up as old friends. Erdogan and Russian President Vladimir Putin seem to be treating the November 2015 shootdown of a Russian Su-24 by Turkish F-16s and the feuding that followed as an anomaly in an otherwise chummy relationship. As Putin said, "Our priority is to bring our relations back to pre-jet crisis level" — basically to get past this ugly episode and have everything go back to normal. If only it were that easy.