Every year the city of Boston receives a gift from the city of Halifax and the province of Nova Scotia - a Christmas tree. Its purpose is to bind the two regions in friendship, but it also commemorates the response to a terrible tragedy that happened more than a century ago. On Dec. 6, 1917, a series of explosions devastated Halifax, killing thousands and injuring many more. Massachusetts responded immediately, before knowing any details, and saved, or relieved the suffering, of thousands more. This is the back story; the saga of the amazing response to those awful events that prompts Nova Scotia's annual present to Boston.