As has been noted on this site in recent weeks, a decisive military victory could be achieved in Syria by the R+6 forces, with the addition of two divisions of Russian troops. The remaining strongholds of both the Islamic State and Nusra could be over-run in a matter of months, under such a beefed-up Russian deployment. Clearly, Russian President Vladimir Putin and his top military commanders are aware of this reality. Yet, so far, despite recent hints of a further Russian military deployment into Syria, no such decisive decision has been taken, begging a fair question: What is holding the Russian President back from bringing the Syrian war to a short-term successful conclusion? Sane military and intelligence circles in the United States would quietly applaud such an intervention, President Obama would make some public pronouncements of complaint, but would not do anything further. Saudi Arabia would sulk and ultimately blame the Obama Administration for "allowing" such an outcome, that flies in the face of their persistent goal of installing a Sunni Salafist regime in Damascus. Even the would-be Sultan of the New Ottoman Empire, President Erdogan, would take it in stride–so long as Syria remained a unified nation without a large portion hived off into an independent Kurdistan.
IMHO, President Putin is holding back for larger strategic reasons that go beyond Syria or the Middle East. Putin knows that there are growing splits within Europe over both the NATO provocations against Russia, which will be advanced at the upcoming NATO heads of state summit in Warsaw in early July. He knows that the European Union will soon be deciding on whether to extend the sanctions against Russia beyond the July 31 expiration date, and that there are a number of important European states where there is diminishing enthusiasm for the sanctions and for the ostracizing of Russia. Putin just visited Greece and offered to invest in a major gas storage facility that would be a tremendous boost for the desperate Greek economy. Italy's Prime Minister Renzi has broken ranks and announced he will attend the St. Petersburg Economic Forum later this summer. European Council President Junker has announced he, too, will attend. Germany's Foreign Minister Steinmeier has issued statements during a recent tour of the three Baltic states, in favor of pulling back from the Russian sanctions, if not all at once. He has linked lifting of the sanctions to "progress" on Russia's part in implementing the Minsk II agreement on eastern Ukraine. Russia has been asked to support a plan for elections in the Donbass region, overseen and policed by the OSCE. Nobody is talking about rolling back the status of Crimea in all of the talk of reducing or terminating sanctions. European nations are divided.
Putin is also in the process of successfully engineering his own "Asia pivot," one that is based more on economic than purely military ties. The recent visit to Russia by Japanese Prime Minister Abe was an important signal that Tokyo is not willing to always follow Washington's lead–when vital Japanese economic and diplomatic interests are at stake. Abe will attend the Far East Economic Forum in Vladivostok this summer at Putin's invitation, and Putin will visit Japan before the end of the year. There is a real prospect that the northern islands dispute, which has been a barrier to close Russian-Japanese economic ties, will be settled sometime in mid-2017, according some well-placed Japanese friends.
In short, President Putin is taking into consideration a complex series of global developments and opportunities. He will act decisively, but not in haste. The Russian military intervention in Syria, launched last September, has stabilized the Assad regime and seen the Syrian Army rebuilt into a far more effective fighting force. Putin has also seen that Iran's effectiveness in Syria has proven to be over-rated. When Russia pulled back some of its military support for the Assad government earlier this year, anticipating that Iran would be able to fill that vacuum, it proved not to be the case. Russia is now quietly back in Syria on a scale equal to the peak of the late 2015 intervention.
It may be fairly debated whether the broader calculations–NATO's pending deployments along the Russian western front, the emerging splits in Europe, the opportunities in Asia–warrant holding back from an all-out decisive offensive in Syria. But it is clearly the case that there are global strategic factors that are in play. Putin is taking the long view and the global view of how Russia can best manage a dangerous and unpredictable world disorder.