Moscow (CNN)It was the largest city in Syria before the war, and for years has been a main rebel stronghold. Now, a defining battle is looming for Aleppo.
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What does Russia want in Syria? 5 reasons Putin backs Assad
Regime forces and their allies on the ground, supported by Russian bombers in the air, are tightening the noose around the eastern half of the city, still held by a coalition of rebel groups.
Aleppo is the regime's most prized target and such regime gains would have been unimaginable for before Russia entered the war late last year.
Since Russia came to President Bashar al-Assad's aid at his request, the Syrian battlefield has transformed rapidly.
But what exactly is Russian President Vladimir Putin up to in Syria? What does Russia really want? Here are five key reasons Russia won't be leaving Syria any time soon.
Russia has significant economic and military interests in Syria, such as a Mediterranean naval base at Tartus, that it is determined to keep. Russia has had a naval facility in Tartus since Soviet times, and although it is has been more of a repair yard and warship supply station, consisting of a "single pier," it is Russia's only base in the Mediterranean. The very real possibility of Assad's regime collapsing is likely to have been the primary reason the Kremlin decided to deploy its powerful air force last September.
Since it entered the war, the strategic center of Russia's military operation has been Hmeymim airbase near Latakia.
Putin is sending a message to the world: Russia is still a force to be reckoned with.
The U.S.-led invasion of Iraq, which deposed long-time dictator Saddam Hussein, and the 2011 toppling of Moammar Gadhafi's Libyan regime -- helped by an international coalition bombing campaign -- left Moscow deprived of key allies.
More recently, Western support for the overthrow of Ukraine's pro-Russia leader Viktor Yanukovych prompted Russia to annex the Crimean peninsula and provide support for pro-Russia rebels in battles in Eastern Ukraine, fighting that mushroomed into the bloodiest Europe has seen since the wars over the former Yugoslavia in the early 1990s.
Syria under Assad is seen by the Kremlin as a key pillar of its strategic influence in the Middle East and Moscow is extremely reluctant to let it go.
Kremlin concerns about the spread of Islamist violence are genuine. Russia has been the target of repeated brutal terrorist attacks carried out by jihadist rebels. Islamist rebels from the southern Russian republic of Chechnya have been fighting for independence since the 1990s, although a brutal six-year campaign by Moscow silenced much opposition and the autonomous region is now firmly under the control of Russian-appointed leader Ramzan Kadyrov. But separatist groups do continue to inflict violence on Russia, like the deadly 2014 bombings in Volvograd.
Russia fears an ISIS victory in Syria would have reverberations at home, as some of the top military commanders of ISIS are Russian speakers of Chechen origin.
More recently, the downing of a Russian passenger jet over Egypt's Sinai Peninsula -- for which ISIS claimed responsibility, perhaps in retaliation for Russia's support of the Assad government in Syria -- has only made Moscow more alarmed at the spread of groups such as Islamic State and al Qaeda affiliates.
It has called for an international coalition to destroy them.
Low oil prices and Western sanctions over Ukraine have plunged Russia's economy into crisis. It shrank by 3.7% in 2015, making it one of the 10 worst-performing emerging markets in the world, the International Monetary Fund calculated. The value of the Russian ruble fell to a record low against the dollar, and that effectively made millions of Russians poorer than they used to be.
President Putin's domestic popular support has remained strong, but with no end in sight to the economic pain, the Kremlin desperately needs a distraction. A Syrian war, as long as Russian casualties remain low, is a way of rallying support and boosting national pride.
From its top-of-the-range Su-35 air superiority combat jets to its brand-new ship-launched Kalibr cruise missiles, Syria provides Russia with a dramatic backdrop to promote its most high-tech weaponry. Russia is already one of the world's biggest arms producers. Sales to nations like China and India -- which are carefully watching the Russian impact on the Syrian war -- could now see a significant upswing.