ON THE MORNING OF DEC. 6, 1917, A NORWEGIAN CAPTAIN WAS STEERING the SS Imo out of the harbor at Halifax, Nova Scotia. The ship bore a banner that read BELGIAN RELIEF, and was on the way to deliver humanitarian supplies to war-torn Europe. Just before 9 a.m., it collided with the SS Mont-Blanc, a merchant steamer bound for Bordeaux.

Sparks flew, setting fire to fuel spilled from barrels lashed on the deck of the Mont-Blanc. Flames soon reached the hold, which contained nearly 3,000 tons of explosives for military use. An enormous blast ripped through the harbor, sending a fireball into the sky and spreading destruction over half a square mile. It was the biggest explosion humans had ever created at that time, and it leveled homes, sank ships, started fires and, in an instant, injured or killed thousands of people.

“For miles around nothing but a flaming inferno,” Frank Baker, a sailor in the Royal Navy, wrote in his diary, “charred bodies being dragged from the debris and those poor devils who were left still lingering were piled into motor wagons and conveyed to one of the improvised hospitals.”

The region was ill-equipped for dealing with a medical emergency on this scale; its hospitals were already filled with soldiers recuperating from war injuries. As word traveled, a response began to muster in Boston, nearly 700 miles away. The first relief train pulled out of Boston’s North Station that evening with supplies and a team of surgeons and 10 nurses, six of whom had trained at Massachusetts General Hospital. After considerable delays because of a massive blizzard, the train arrived in Halifax two days after the blast. Meanwhile, Harvard Medical School, in conjunction with the Red Cross, packed a second train with enough staff and equipment to supply a 500-bed hospital.

The Boston contingent relieved their exhausted colleagues and transformed damaged schools and clubs into hospitals to treat the injured. With the help of U.S. sailors, they restored and cleaned an officer’s mess called the Bellevue, hoisting an American flag outside. In their first day of operation, Bellevue took 60 patients; the next day, they admitted 100.

As first aid gave way to long-term care, hospitals began specializing for certain patients. Harvard’s William Ladd, who completed his general surgical training at MGH, set up in St. Mary’s Boys’ School and mainly treated children—the unit was later recognized as a pioneering surgical ward for pediatric patients. Ladd was the last doctor to return to Boston, and advanced the understanding of how trauma care for children is different from that for adults.

The burning ship also attracted the attention of people on shore, who watched the disaster unfold from their windows. When the blast from the explosives occured, it shattered the windows and propelled tiny shards of glass into the eyes of onlookers. As a result about 40 people lost both eyes and approximately 140 lost one eye. G. H. Cox, a Canadian surgeon, who said he had operated for so long that his knives began to dull, described one patient’s eyeball: “It was as if the ball had been laid open and then stuffed with pieces of glass—or sometimes crockery, brick, splinters, and on palpation they would clink.” The Massachusetts-Halifax Relief Committee paid for 145 artificial eyes and at least 300 pairs of eyeglasses.

In the end, the disaster caused the death of nearly 2,000 people, and injured 9,000, altogether nearly a quarter of the city’s population. It also prompted the first academic studies of civilian disaster management. In 1920, a paper by Samuel Henry Prince, a sociologist and priest who helped assist Halifax survivors, examined the importance of triage in civilian catastrophes.

One of the Halifax explosion’s more enduring legacies may be the lasting bond it created between the two cities. Boston not only sent physicians and medical supplies but also opened a warehouse to provide free household goods to residents of Halifax, many of whom were now destitute and facing a hard winter. In all, Massachusetts sent $750,000 in aid, the equivalent of more than $15 million today.

The citizens of Halifax have never forgotten. In 1971, the province of Nova Scotia began sending the city a Christmas tree every year to be displayed in Boston Common. When Boston faced its own tragedy in 2013, with the bombing at the finish line of the Boston Marathon that killed three people and injured several hundred, they sent a donation of $50,000 to MassGeneral Hospital for Children’s Pediatric Palliative Care Program.

For the 100-year anniversary, several Canadian institutions have joined together and launched a program to tell 100 stories about the Halifax disaster. They describe rescue efforts, unsung heroes and, in more than one case, the profound medical care that was given in a time of great need.