Syrian troops are on the brink of surrounding the rebel-held portion of Aleppo, a major setback for the opposition, which has increasingly found itself on the defensive and losing territory in the war.

The outgunned antigovernment fighters are pessimistic about their ability to stave off an advance by troops in the northern city. A siege would cut off an estimated half a million people from access to food, medicine, fuel and other basic necessities.

For more than two years, Aleppo, once Syria’s commercial hub, has been divided between rebel and government forces. Only a small stretch of land, less than a mile wide, and one road connect the opposition-held portion of the city to the countryside and highway leading to Turkey. […]