Russian President Vladimir Putin announced on March 14 that he would begin withdrawing his main forces from Syria. Russia appears to have deployed about 70 aircraft of a variety of types and more than 4,000 support personnel to protect and maintain the aircraft. It was not a major deployment, but it shifted the situation on the ground rapidly. Before the deployment there had been serious discussion that Bashar al-Assad's regime had its back against the wall. That expectation dissolved as Russians carried out attacks against those working to overthrow the regime.
It is unclear precisely why the expectations shifted. It is possible that the limited number of sorties the Russians could fly was sufficient to break the operational capabilities of the opposition. It is possible that simply the Russian presence was enough to shift the psychology of the opposition and break their will. It is also possible that the opposition was so fragmented and so fundamentally weak that virtually anything would shatter them. This can be discussed endlessly, but the fact is that the Russians came in and achieved the outcome they wanted.
The question of course remains: Why did the Russians intervene in the first place? Assad's father had been close to the Soviets, and post-Soviet Russia made gestures at continuing the relationship. But Syria was never central to Russian interests, and having any number of other problems, particularly Ukraine, devoting precious resources to solving what from Russia's perspective was a relatively small problem is odd. But when you think about it, it made complete sense, even beyond ensuring Assad's survival.
The first reason Putin intervened in Syria was simply to show that he could. He had two audiences for this: the Russian public and the West, particularly the United States. Russia's performance in Ukraine was mediocre at best. It "seized" Crimea against no opposition and encouraged an uprising in the east that failed to ignite the region. Its intelligence service failed to understand what was happening in Kiev and failed to shape it. And even more important, the plunge in oil prices created a massive economic crisis in Russia. It was a critical moment for Moscow domestically and in its foreign relations.
Deploying an air wing consisting of different kinds of aircraft and then maintaining them in combat operations for months demonstrated that Russia had a significant military capability and was able to deploy it effectively. In Russia, as in other countries, successful, short military operations generate massive support. It demonstrated to the United States that it had the ability and will to intrude into areas that the United States regarded as its own area of operations. It changed the perception of Russia as a declining power unable to control Ukraine, to a significant global force. Whether this was true was less important - it needed to appear to be true. And it cannot be denied that there was truth to it.
The second point is much stranger and not fully aligned with the prior reason. The Russians intervened in Syria in order to bail the United States out of a very difficult situation. The United States opposed the Assad regime and wanted it replaced by a coalition of opposition forces. It was increasingly obvious that this was not going to happen. Assad might fall but what would replace him was a fractious opposition as much at war with each other as with Assad. This might be preferable to Assad, but the Islamic State was deep into Syria and had already engaged and defeated some of Assad's armored forces - not to mention that IS controls far more territory than any other rebel group. If Assad fell, and if he was replaced by the opposition, it was conceivable they could in turn be replaced by IS. The U.S. was aware that it had constantly underestimated IS, and the possibility of IS in Damascus was both real and unacceptable to the United States.
The United States had a political problem. Not only had it opposed Assad, it had been deeply aligned with anti-Assad factions. It could not suddenly become the protector of the Assad regime. At the same time, the United States, at that moment, could not afford the fall of Assad. The Russian intervention solved the problem for the United States. Assad was saved. IS was blocked and a situation that was spiraling out of control was contained.
Was this a formal deal or merely the unexpected outcome? I doubt that papers were signed but I also doubt that it was unexpected by either side. The Russians certainly knew the American situation in Syria: the U.S. didn't trust its own sponsored opposition, was unnerved by IS and helpless to do what it had to. The Russian intervention followed directly from Moscow's public position and posed no problem for it.
By doing this, in the face of massive American air power, Russia either assumed that it could coordinate with the United States in time or that coordination was discussed in the beginning. The solution to the American problem in Syria is one of those things that you find out about 50 years later when documents are declassified. I am not saying there was an agreement. I am saying that, agreement or not, the Russians knew they were solving an American problem, and the Americans, for all their rhetoric, knew their problem was being solved. And that bought the Russians some points on their second biggest problem.
Their biggest problem is of course oil, for which there is no solution. Their second biggest problem is Ukraine, a fundamental interest of the Russians, which they cannot permit to become part of the Western alliance system - a matter we have extensively discussed. The core Russian interest is the military neutralization of Ukraine. Their secondary interests are some degree of autonomy in the east and some settlement on Crimea that gives Russia more extensive rights there than it had before.