BEIRUT — The bravado sounded familiar. Like the leaders of other countries swept away by Arab Spring uprisings, President Bashar Assad of Syria vowed to never be forced into exile and to die in his homeland.

Assad dug in his heels even as world powers moved to boost the opposition in Syria’s civil war, the latest turn in a nearly 20-month-old crisis so overwhelming that even the Red Cross says it can no longer cope.