Although not initially targeted in the Syrian uprising, the fight for the city of Aleppo soon became the key battleground in the bloody conflict, which has left 300,000 people dead and millions displaced.
Here's the battle explained, including who fought it, how it became a humanitarian crisis and what comes next.
Why is the battle for Aleppo so important?
The four-year battle for Aleppo is often referred to as Syria's Stalingrad, a war of attrition whose victory will be as symbolic as it is strategic in the complex multi-front conflict.
The ancient city was previously the country's industrial and financial hub, home to more than 2.3 million with an Old City listed as a Unesco World Heritage site.
When did it start?
Syrian rebels attacked Aleppo in July 2012, a year after the 2011 uprising that spawned from anti-government protests against President Assad before spiralling into civil war.
The rebels gained control of the city's east but government forces held firm in the city's western districts.
Though the contours of the battle frontline changed, neither side could make decisive gains over the last four years, largely thanks to being propped up by their outside supporters.
Who is involved in the conflict?
Pro-Assad forces have drawn the most support from Russia and Iran, but the government is also backed by other Shia fighters from Afghanistan and Iraq and Lebanese Hezbollah forces.
Each holds their own motivations for preventing Assad's overthrow.
The rebel opposition, led by the Free Syrian Army, is supported by predominantly Sunni rivals and receives financial aid from the US, Saudi Arabia and Turkey, supported by Western nations who want to see Assad overthrown.
The rebels are also backed by hardline Islamists, including the group formerly known as the Nusra Front, now called Jabhat Fateh al-Sham.
Russia claims it wants to see the pro-Assad state prevail to prevent Islamic extremists, including IS, from gaining a presence in Aleppo and the rest of the country.
What has tipped the balance in the battle?
Russia's daily air strikes have emboldened pro-Assad fighters, helping them bombard eastern Aleppo quarters and besiege the rebels.
From mid-2016, it saw the pro-government forces gain the upper hand, snapping a key supply route into the rebel-held east and trapping more than 250,000 in the area.
Though the rebels established a new supply corridor it was quickly overturned and the siege restored, paving the way for the pro-government forces' incremental percentage gains of rebel-held districts.
Why was the Russian bombardment so controversial?
Moscow stands accused by Western powers of intentionally hitting civilian areas, in particular hospitals and schools, despite pledges to only target militants since beginning airstrikes in September 2015.
A year later Foreign Secretary Boris Johnson said Russia was guilty of prolonging the conflict and may have committed a war crime by targeting an aid convoy.
The siege in the city predominantly trapped civilians, leaving them as trapped pawns in the conflict as rebel-held enclaves were squeezed.
What has been the response from the international community?
Despite repeated statements of condemnation and accusations of war crimes, the international community has failed to protect civilians or broker a peace agreement.
Rounds of diplomatic talks led by US and Russia have failed to agree on a common strategy to end the war and only succeeded in establishing brief pauses in the conflict.
Donald Trump's impending arrival in the White House could challenge the UN-Russia dynamic that underpins the conflict.
How did the battle become a humanitarian crisis?
Since 2015 Amnesty has accused both sides in the conflict of committing war crimes and making Aleppo "increasingly unbearable" for civilians.
The horrors of Aleppo have been near ceaseless while the image of a young boy, Omran Daqneesh, dazed and bloodied from an attack, intensified condemnation after gaining worldwide attention in September.
He was among many children filmed being pulled from the rubble of airstrikes this year.
The subsequent siege led to serious food and fuel shortages and repeated pleas were made for evacuation for medical emergencies.
Although a unilateral pause in mid-October allowed civilians and rebels to leave most stayed.
Rebels then stood accused of preventing civilians from leaving, holding them hostage as human shields.
How many civilians have been killed or forced to flee?
An estimated 20,000 civilians have been killed in the four-year battle, according to the British-based Syrian Observatory for Human Rights
The estimated number of Aleppo residents to have been killed in the conflict.
The estimated number of residents killed in rebel-held areas.
The Syrian Observatory for Human Rights estimates at least 415 civilians have been killed since the government assault intensified in mid-November.
Thousands of civilians have fled as districts fell into government hands. The UN has said hundreds of men have also gone missing.
What happens next?
The fall of Aleppo would be the worst rebel defeat since Syria's conflict began in 2011, leaving pro-government forces in control of the country's five major cities.
Assad's forces will celebrate a symbolic recapturing of what was once the nation's economic centre, giving a psychological edge.
President Assad has previously said the recapturing of Aleppo "won't mean the end of the war in Syria, but it will be a huge step toward this end".
But experts do not expect the conflict to end quickly as his forces bid to claim the remaining rebel strongholds, particularly the region of Idlib, while trying to protect their gains.
Though the main cities are in government hands, large swathes of the country remain in the hands of rebel forces and Kurdish forces, with jihadist groups (including Islamic State) poised to exploit weaknesses.
Recent attacks in Homs and Palmyra proved the government-held territories are far from solid, underlining how difficult it will be to achieve a resolution in the catastrophic conflict.