Adolf Hitler’s aggression created an alliance of Great Britain, the United States, and the Soviet Union that brought down the Third Reich in flames. However, that “Grand Alliance” consisted of countries with very different aims united only by the German danger. As soon as victory came in sight, the allies began to fall out with one another. Their competition produced the Cold War.
Now the same thing is happening as the ISIS caliphate begins to crumble. The current wars in the Middle East (the ISIS war, the Syrian civil war) have become proxy wars. Turkey has become the chief supporter of the various Sunni Arab rebel groups, like the Free Syrian Army; the Russkies and the Iranians are the supporters of the Assad regime; and the Americans are the chief supporters of the Kurds in both Iraq and Syria. Now, these disparate allies-of-convenience are beginning to pursue their interests. Their proxies are likely to pay the price.
The central dynamic in the next phase is likely to be Kurdish nationalism. The Turks hate the Kurds, and the Kurds hate the Turks. Turkey is a NATO member (if not exactly an ally), but the Americans have supplied the Kurds with a lot of support. So, at some point, the Americans are going to have to make a choice or broker a deal. Now the Kurds have begun to doubt American support. The Syrian Kurds, at least, have had some contact with the Russians.
Turkish support for the Sunni Arab rebels actually puts them on the side of the major losers in this struggle. Both the American-backed Kurds and the Russian-backed Assad regime have greater assets on the battle field. Contacts have opened between the Assad regime and the Syrian Kurds. The short-term goal of such talks might be co-operation against ISIS, but the long-term goal might be a meeting of minds about Turkey. Naturally, Turkish president Erdogan would rather cut a deal with the Assad regime he has been trying to overthrow in order to forestall an Assad-Kurd alliance. Assad’s chief aim seems to be to get control of the key western parts of Syria, where the Sunni rebels are his chief opponents. The Sunni rebels—commonly called the “moderates” by President Obama—are going to pay a heavy price if this happens.
For its part, Russia is allied with Iran to support the Assad regime. Now the Iranian-controlled militias fighting in Syria have ignored Russian-sponsored local truces. Both the Russians and the Assad regime are going to have to choose whether to cut ties with Iran.
Their immediate problem is that they want to know what the Americans are going to do. In so far as Syria is concerned, the Trump administration, like the Obama administration, sees things almost entirely in military terms. They want ISIS destroyed. This has produced a pause in American participation in Syrian peace talks now underway in Geneva. At the same time, the American face a dilemma: the Trump administration wants to improve relations with Russia, the Russians are allied—for the moment—with Iran, and the Trump administration is hostile to Iran (as are several of America’s regional allies). The U.S. and Russia recently joined to block an attack by Turkish Sunni clients toward the ISIS stronghold in Raqqa because it would have cut across a movement by Kurds and Assad forces. Does this have any longer-term meaning?
So, who will get eastern Syria once ISIS is destroyed? The Kurds?
 Yaroslav Trofimov, “Battle for Raqqa Set to Shape Mideast,” WSJ, 10 March 2017; Yaroslav Trofimov, “ U.S. Disengagement Creates Hurdles for Syria Peace Talks,” WSJ, 3 March 2017.
 That is, Syria may be headed toward “de facto” partition.
 An American tradition. Look at Stephen Ambrose, Eisenhower and Berlin, 1945: The Decision to Halt at the Elbe. (1967).
 To make matters worse, the out-of-power Democrats want to preserve the deal with Iran brokered by John Kerry while also attacking Russia as a way of impugning President Trump.