ALEPPO, Syria—In late March, I arrived in Aleppo as the battle between rebels and regime forces for control of the city was in its 21st month, one of the longest and bloodiest campaigns of Syria's civil war. The evening flight from Damascus aboard a small Russian-made, low-winged Yakovlev Yak-40 was uneventful, though many of the passengers' bags, including mine, were left behind because the plane couldn't carry all the weight. The real challenge was to get from Aleppo International Airport to the regime-held west side of the city.

The...