I should like to intervene in this discussion, as a British-Syrian woman, I follow the developments in Syria very closely, as I am sure everyone here can imagine.
Russia has only partially pulled out, we are only talking about a handful of its forces, its naval base and air base not only remain in-tact but have been expanded. Russia military advisers remain in place and Russia military observers will remain on the ground to 'monitor' the partial ceasefire that has been in place for less than a month.
Russia focused on hitting anti-ISIS rebels and civil Syrian opposition movements, according to the Pentagon, 80% of the air strikes were against the Syrian opposition and not ISIS. Some of the rebel networks hit by the Russians were keen American allies and people who the Americans believed they could work with- but Russia has significantly weakened them- not defeated them.
Its not secret that Russia has saved the Assad regime from collapse in the immediate term, but even they realize that his regime can never take back the whole country, they only realized the dismal state the Syrian regime was in once they intervened. They were even more alarmed to hear Assad saying he will take back the whole country, I think the partial withdraw is a signal to Assad that Moscow does not share this ambition because they don't think he's capable of it. There are also economic reasons for this pullout too, Russia's dismal economy would be even worse hit, if they get bogged down in Syria.
The real aim for the Russians was the European Union. Prior to the partial ceasefire, Russian airstrikes against civilian targets in places like Aleppo (which included hospitals, schools, markets, energy plants etc) greatly increased the numbers of refugees, many of whom would make their way to Europe, placing greater burden on European member states already dealing with a deadly cocktail of mass refugee crisis, terrorism and the rise of xenophobic reactionary politics. Different European States are blaming each other and are unable to form a unified block against the Russians, which greatly increases Russia's political power.
On ISIS, the so-called 'retreat' of ISIS has been overstated, and there is a clear separation between Iraq and Syria in terms of how the 'fight' is going. ISIS has lost more in Iraq than in Syria. But on Iraq, we have to bare in mind a few things here, the figure of how much territory ISIS has lost is likely to be inaccurate. The Iraqi government is dishonest about its progress against ISIS, if often announces victories that have never happened, and it often expands its definition of who is ISIS against Sunnis who oppose the central Iraqi government. That said, it did have considerable success in Ramadi, but only when it allowed local Sunni forces to lead the fight and forbade Shi'i militia from the front line. This wasn't only a condition for Sunni support it was also an American condition who refuse to launch air strikes against ISIS areas, if Shi'i militias are involved in fighting.
The Syrian front has seen even less success, the Kurdish PYD faction, has had some success in pushing out ISIS forces in Kurdish majority areas, but a pinch of salt is needed here, as people keep overstating the PYD's fighting prowess. By enlarge, the Americans do most of the heavy lifting, they launch strikes against ISIS targets, ISIS stages a tactical withdrawal from the area to avoid being picked off by the Americans, and after their main forces have pulled back, the PYD enters the area and claims victory. But without the Americans this would not have been possible.
On Assad regime, he cannot win in Syria, he simply doesn't have the forces to regain control of the country. Aleppo's route to Turkey was not cut off by the Syrian army but by foreign Shi'i militia and Hezbollah. In truth, Assad's forces are depleted, they are anywhere between a third to a fifth of their 2011 strengthen in terms of their manpower. Much of that army the regime doesn't trust in any case, often the planning is done by Iranian generals and they issue orders to whoever they decide.
This is the situation as it stands.