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If government-allied troops fail to recapture the city, questions will be raised about their ability to win the war with force. It would also strengthen the hand of Islamist militant fighters inside the armed opposition, who have played the most important role in the defense of the area.

Who is fighting there?

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Pretty much everyone. On the government side, Assad's forces are heavily supported by Iranian troops and Iran-backed militias, as well as by Russian warplanes and commanders. The rebel groups also fight in a coalition, one dominated by hard-line Islamist forces including Jabhat Fatah al-Sham, Ahrar al-Sham and Harakat Nour al-Din al-Zenki. But more moderate forces remain, some of them receiving U.S.-approved weapons.

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Aleppo is a city divided. What are conditions like in the east and west?

The eastern, rebel-held districts have been subject to a crippling siege since July, leaving their 250,000 residents with severe shortages of food, medicine and fuel. Streets have been shattered by four years of government — and, more recently, Russian — airstrikes. Civilian areas and hospitals have been systematically targeted in the strikes. The area resembled an apocalyptic wasteland in recently released drone footage.

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In government-held western Aleppo, civilians are also suffering from shortages — albeit less severe — and civilians close to the front line live in fear of rebel shelling.

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How many people have died in the latest offensive?

Reliable numbers are hard to come by. In eastern Aleppo, the White Helmets rescue group said at least 300 civilians have been killed and more than 820 wounded since Nov. 15. More victims were lying under the rubble, out of reach until the bombing stopped. In western Aleppo, the Britain-based Syrian Observatory for Human Rights said Tuesday that at least 16 civilians — 10 of them children — were killed by rebel shelling.

Is a military solution the only option for Aleppo's warring parties?

The U.N. envoy for Syria, Staffan de Mistura, has proposed a political solution — rebel fighters leave eastern Aleppo through safe exits, government and Russian forces stop the bombing, and a council from the opposition-held districts takes control of the area. But the Syrian government has roundly rejected this. According to one minister, retaking Aleppo in full is now a key military priority.