Why the Americans might let the Russians win in Syria

The Americans might let Russia win in Syria.
Why?
Essentially, because they have very little to lose by such an outcome.
Let us imagine, for the sake of example, that the Assad regime, with the significant support of the Russians, regains control of all of Syria.
How bad is that for the USA?
Not very, is the simple answer. If Assad regains control over Syria it is merely a return to the state of play before the Syrian Civil War began. The US will be no worse off than they were before the fighting began. Not only that, the refugee crisis, which is causing so many problems for their allies in Europe will be brought to an end, and the Islamic state would be suppressed as well, at least in Syrian territory.
The Americans lose very little by this scenario.
Admittedly, the Russians might be able to boast about having outplayed the Americans and saved the Europeans from a refugee crisis. They may well strengthen their reputation with many European and Middle Eastern parties as a result, but the USA will remain the world’s super power and no-one is about to forget that just because the Russian’s performed well in a theatre where the Americans had no vital interest to push.
 
Prevention, not Cure
America’s main objective in Syria at the moment is not so much to ‘achieve’ something as to ‘prevent’ something.
I have heard it said in reference to the American involvement in Syria that, “They don’t have a dog in this fight.” That is very true, but I would contend that at the moment America’s main objective in Syria is not keeping their dog in the fight, but rather trying to keep their dog out of one. In this context the metaphorical dog which the US is trying to keep out of the fight is Turkey.
Turkey is a valuable ally who controls the seaway between the Black Sea and the Mediterranean. The USA sees Turkey as a strategic check to Russian influence and military power being extended into the Middle East and Eastern Mediterranean. Turkey is a member of NATO and a key part of the USA’s cordon of allies on the western border of Russia. The USA does not want to lose this ally.
However, Turkey is barely managing to control the independence aspirations of its 15 million strong Kurdish minority as it is. American strategist are dead scared that if Turkey puts troops on the ground in Syria they will get into conflict with the Kurdish forces there. The Kurds are the only successful fighting force in the Syrian conflict that most Western countries feel they can back. Indeed, by the values of the western media the Kurds, both in Syria and Iraq, are the only heroes of their respective wars.
If the Turks do something nasty to the Kurds, then the Americans may feel that they have no choice but to withdraw their support from this valuable ally. Turkey could be suspended from NATO, and from that point there are any number of scenarios which could play out, all of which have Turkey travelling a very difficult road. All of those roads would involve a Kurdish insurgency within Turkey and many of them would involve Russia throwing its support behind an independent Kurdistan in a situation where Turkey is abandoned by the majority of the West.
To play that scenario out further, we might then have Turkey losing a quarter of its territory to a new Kurdish state that is strongly allied to Russia. Russia’s influence in the region would have grown massively, and not only that but it would be seen in Europe as a liberator of the oppressed.
This would be a great victory for Russia in its strategic competition with the USA. Compared to such an outcome, letting Assad resume control of Syria would certainly be seen by American strategists as the lesser of two evils.
 
What about the Saudis?
Before the Russians got heavily involved the Americans, and the Saudis themselves, might have been imagining that Saudi troops were the logical forces to finally occupy the territories currently ruled by the Islamic State. In American eyes the Saudis would still be preferable to both Turks and Iranians, but now that the Russians are heavily invested on the ground and declaring that Saudi troops in Syria could spark a “new world war”, to quote Lavrov, the stakes have risen substantially and what once might have looked likely now seems far less so, at least in the short term.
The Americans will not be too put out by this. Saudi Arabia would become massively more powerful were they to be the final troops on the ground within the Islamic State’s territory and the Americans must have had misgivings about that.
 
American Troops?
America is unlikely to put its own troops on the ground in Syria; there is so little to be gained from such an action. It does not profit the US to invade Syria. They will, however, continue to stay involved in the conflict. By staying involved they can work to see that their allies do not make disastrous choices and they will be on hand to capitalise if their opponents miss-step. In many ways the US is in an enviable position in relation to this conflict simply because it doesn’t have to be there. Even the worst case scenarios would be more embarrassments than disasters for them.
 
The USA will stay involved in the Syrian Civil War and try to get the best outcomes it can, but at the end of the day it may decide that letting Russia help Assad to reconquer Syria could be the least worst option.