Syrian rebel forces were on Wednesday within six miles of the central city of Hama, a city freighted with past horrors for antigovernment forces. Syria’s fourth-largest city might represent an even more significant fillip than the sudden capture last week of Aleppo — the country’s most populous city — in the civil war's most significant offensive in years and one that could dramatically shift the balance of the country's conflict. Despite the map of Syria having been repeatedly redrawn since 2011, Hama has never been in rebel hands. Insurgents and Turkey-backed fighters opposed to the government of Syrian President Bashar...