The bombs at the Boston Marathon were designed to maim and kill, and they did. Three people died within the first moments of the blast. More than a hundred and seventy people were injured. They had their limbs blown off, vital arteries severed, bones fractured, flesh torn open by shrapnel or scorched by the blasts’ heat. Yet it now appears that every one of the wounded alive when rescuers reached them will survive.

Medically speaking, this is no small accomplishment. We’ve seen bombs like this in the battlefields of the Middle East, but rarely in cities like Boston. In the past century of wartime conflict, explosive devices have escalated to become the predominant cause of military casualties. Among American personnel wounded in our wars in Iraq and Afghanistan, they have accounted for three-quarters of injuries; gunshot wounds for just twenty per cent. It has been an historic accomplishment for military medical units to bring case-fatality rates from such injuries down from twenty-five per cent in previous conflicts to ten per cent today. And according to data from the Israeli National Trauma Registry, explosives used in terror attacks have tended to be three times deadlier than those used in war—because civilians don’t have armor, because victims span a wider range of age and health, and because preparedness tends to be less systematic. Nonetheless, in Boston, they survived.

How did this happen? Something more significant occurred than professionals merely adhering to smart policies and procedures. What we saw unfold was the cultural legacy of the September 11th attacks and all that has followed in the decade-plus since. We are not innocents anymore.

The explosions took place at 2:50 P.M., twelve seconds apart. Medical personnel manning the runners’ first-aid tent swiftly converted it into a mass-casualty triage unit. Emergency medical teams mobilized en masse from around the city, resuscitated the injured, and somehow dispersed them to eight different hospitals in minutes, despite chaos and snarled traffic.

My hospital, the Brigham and Women’s Hospital, received thirty-one victims, twenty-eight of them with significant injuries. Seven arrived nearly at once, starting at 3:08 P.M. All required emergency surgery. The first to go to surgery—a patient in shock, hemorrhaging profusely, with inadequate breathing and a near-completely severed leg—was resuscitated and on an operating table by 3:25 P.M., just thirty-five minutes after the blast. The rest followed, one after the other, spaced by just minutes. Twelve patients in all would undergo surgery—mostly vascular and orthopedic procedures—before the evening was done.

This kind of orchestration happened all across the city. Massachusetts General Hospital also received thirty-one victims—at least four of whom required amputations. Boston Medical Center received twenty-three victims. Beth Israel Deaconess Medical Center handled twenty-one. Boston Children’s Hospital took in ten children, ages two to twelve. Tufts Medical Center and St. Elizabeth’s Medical Center each treated eighteen victims. One emergency physician told me he’d never heard so many ambulance sirens before in his life.

There’s a way such events are supposed to work. Each hospital has an incident commander who coördinates the clearing of emergency bays and hospital beds to open capacity, the mobilization of clinical staff and medical equipment for treatment, and communication with the city’s emergency command center. At my hospital, Stanley Ashley, a general surgeon and our chief medical officer, was that person. I talked to him after the event—I had been out of the city at the time of the explosions—and he told me that no sooner had he set up his command post and begun making phone calls then the first wave of victims arrived. Everything happened too fast for any ritualized plan to accommodate.

So what did you do, I asked him.

“I mostly let people do their jobs,” he said. He never needed to call anyone. Around a hundred nurses, doctors, X-ray staff, transport staff, you name it showed up as soon as they heard the news. They wanted to help, and they knew how. As one colleague put it, they did on a large scale what they knew how to do on a small scale. They broke up into teams of six or so people, one trauma team for each patient. A senior nurse and physician stood at the door to the ambulance bay triaging the patients going to the teams. The operating-room director handled triage to, and communication with, the operating rooms. Another staff member saw the need for a traffic cop and began shooing extra clinicians into the waiting room, where they could stand by to be called upon.

Richard Wolfe, the chief of the emergency department at Beth Israel Deaconess Medical Center, told me he had much the same experience there. Of twenty-one casualties, seventeen were serious and seven required emergency surgery. One patient came in with both legs almost completely amputated already. Another’s leg was too mangled to save. Numerous victims had open, bleeding wounds, with shrapnel and shards of fractured bone. One had a lung injury from the blast. Another was burned on over thirty per cent of the body. One had to have an eye removed. Wolfe arrived in the emergency department expecting to take charge of assigning everyone responsibilities.

“But everybody spontaneously knew the dance moves,” he said. He didn’t have to tell people much of what to do at all.

I spoke to Deb Mulloy, the nurse in charge of our operating rooms that afternoon, and a few of the other nursing leaders to find out how they knew the dance moves. Mulloy began mobilizing as soon as she saw the news flash onto a television screen. Others learned through Twitter, text messages, smartphone news apps. They all began to act before the alarm had been sounded.

“We just knew this was real,” Mulloy said, “and a lot of people could be hurt.”

Change of nursing shift is at three o’clock. So she immediately notified the day shift to stay on. No one wanted to leave, anyway. This doubled the available staff.

The nurses put all scheduled surgery on hold and began readying eight rooms. They ordered equipment trays for vascular and orthopedic procedures to be brought up from stock supply. They called an orthopedics-manufacturer representative for extra hardware to be mobilized. They got in touch with the blood bank, which was already securing blood from other states. They communicated with other operating rooms around the city to make sure they had enough supplies of equipment, too.

How did they know to get eight rooms ready, I asked. And how did they know to get them ready for vascular and orthopedic procedures? “Did someone tell you?”

“No,” said Brenda McKonly, one of the senior nurse leaders. She just saw the descriptions of the explosion like everyone else, made a surmise about the injuries, and recognized that they needed to get as many rooms ready as they could. To be on the safe side, the staff also got equipment for one room to be ready for a neurosurgical injury and another for a thoracic injury. But as word filtered down from the emergency department, it became clear that their original surmise was correct. All eight rooms would be required, and nearly all the cases involved vascular and orthopedic injuries.