The Green Door sat at the bottom of a flight of iron stairs on Wabash Avenue, tucked between a Chinese laundry that never opened and a pawnshop whose windows held the sad debris of a thousand broken dreams. You knocked three times slow and two fast, and a panel in the door slid open to reveal a pair of eyes and the muzzle of a Colt revolver. If the eyes recognized you and the revolver did not object, the door opened onto a long narrow room filled with cigarette smoke and jazz and the smell of bathtub gin so raw it could strip paint from a Buick.

Tommy Callahan owned the Green Door. He was thirty-four years old, built like a dockworker, with hands that had loaded trucks at the Union Stockyards before Prohibition had made honest work less profitable than dishonest work. His face was a map of old fights: a scar across his left eyebrow from a broken bottle, a nose that had been reset twice and still listed slightly to port, a smile that was missing two teeth on the right side and made him look perpetually amused at something that nobody else could see. Men called him Two Tone because of the grey streak that ran through his black hair like a skunk stripe, a mark he had carried since the age of nineteen when a cop had cracked his skull with a nightstick during the race riots of nineteen ten.

The Green Door was not a fancy joint. The tables were scarred oak, the chairs were mismatched, the stage was a platform of pine planks that creaked under the weight of the musicians, and the liquor was whatever Tommy could get his hands on that week, the chemical composition of which varied from barely drinkable to actively dangerous. But the music was good. Tommy paid his musicians in cash and gin and the promise that if the police raided the place, there was a trapdoor behind the bar that led to a tunnel that led to a coal cellar three buildings over. It was not the Ritz, but it was home.

Mae Callahan ran the bar. She was Tommy's wife of eleven years, a woman with red hair that she kept bobbed short in the new style and green eyes that could stop a drunk in his tracks from twenty paces. She had grown up in the same tenement as Tommy, had married him when she was seventeen and he was twenty-three, and had spent the intervening years building a life out of the scraps that the world had left them. She kept the books, ordered the supplies, paid off the cops, and knew every regular by name and drink preference. She was pregnant with their first child, though only Tommy knew this, and she carried herself with the careful watchfulness of a woman who had something precious to protect.

The night the trouble started was a Thursday in late October. The weather had turned cold early that year, and the wind off Lake Michigan cut through coat and skin and bone like a knife through butter. Inside the Green Door, the furnace was rattling and the crowd was thin and the musicians were taking their break, smoking hand rolled cigarettes and arguing about baseball. Tommy was behind the bar helping Mae polish glasses, a task he hated but performed with the grim diligence of a man who knew better than to argue with his wife when she was in her third month and queasy from the smell of gin.

The door opened and Leon Washington walked in.

Leon was seventeen years old and he carried a cornet case that was held together with string and hope. He was a neighborhood kid, the grandson of a Pullman porter, with cheekbones you could cut glass on and fingers so long and thin they looked like they belonged on a piano player rather than a horn man. He had been hanging around the Green Door for six months, sitting in with the band when the regular cornet player was too drunk to show up, playing with a tone so pure and sweet that it made Tommy think of honey dripping off a spoon.

Mr. Callahan, Leon said, setting his case on the bar. I got a new one. I been working on it all week. You mind if I try it out?

Tommy glanced at Mae. She shrugged, which meant she did not mind as long as it did not cause trouble. Tommy grinned and waved toward the stage. Go ahead, kid. The floor is yours.

Leon climbed onto the pine platform and unpacked his battered cornet. The house band drifted back from their break, curious to see what the kid had cooked up. The drummer tapped out a soft beat on the hi hat. The piano player found a key and held it. And then Leon Washington put the horn to his lips and began to play.

The melody was mournful and sweet, a blues progression in B flat that climbed and fell like a man walking up a flight of stairs and then tumbling back down. It was a song about rivers, Tommy thought, or maybe about trains, or maybe about the long slow ache of a love that had gone wrong and could never be set right. It was the kind of music that made you want to drink more and talk less and remember things you had spent years trying to forget.

The dozen or so customers in the bar stopped their conversations and listened. Mae stopped polishing glasses. Tommy stopped breathing.

The song ended. There was a moment of perfect silence, and then the room erupted in applause.

What is that one called? Tommy asked.

Leon grinned, his teeth white against his dark skin. Blue River Blues. Wrote it myself.

Play it again, someone shouted from the back of the room. And Leon did.

The trouble did not start that night. It started the next morning, when a man named Eddie the Fish Corrigan walked into his boss's office on the North Side and said the wrong thing at the wrong moment.

Eddie the Fish was a low level soldier in the Moran gang, a man whose primary skills were driving fast and shooting straight and not asking questions. He had been in the Green Door on Thursday night, nursing a glass of gin and waiting for a meeting that had been delayed, and he had heard Leon Washington play Blue River Blues. He had liked it. He had liked it enough to mention it to his boss, a lieutenant named Frankie Scales who was having a bad morning because a shipment of Canadian whiskey had been seized by Prohibition agents and his ulcer was acting up and his wife had discovered his mistress.

What do you mean, a new blues song? Frankie said. At the Green Door?

Yeah, Eddie said. This colored kid playing cornet. Real pretty. Called it Blue River Blues.

Frankie Scales stopped rubbing his stomach. His eyes narrowed. Blue River Blues, he said slowly. That is Dean O'Banion's song.

Dean O'Banion had been dead for almost a year, murdered in his flower shop on State Street by men working for Al Capone. But his songs, the melodies that the North Side gang claimed as their own territorial markers, were supposed to remain untouched. Blue River Blues had been written for O'Banion by a piano player who had died in the influenza epidemic, and the North Side crew had been playing it at every funeral, every wake, every memorial gathering since their boss had been gunned down. It was sacred to them, in a way that only gangsters who had lost their leader could understand.

And now some colored kid was playing it in a South Side speakeasy, passing it off as his own, turning a memorial anthem into entertainment for strangers. Someone, somewhere, had taught the kid that song. Someone had violated the unspoken boundary between the territories. Someone was going to pay.

Find out who told that kid the song, Frankie said. And send a message.

The message arrived three nights later.

Tommy was in the alley behind the Green Door, supervising the unloading of a shipment of gin that had come up from a distillery in downstate Illinois, when a black Ford sedan pulled up at the mouth of the alley and blocked the exit. Four men got out. They wore dark suits and dark hats and carried baseball bats wrapped in barbed wire, which was a North Side signature as recognizable as a calling card.

Tommy Callahan? the lead man said.

Tommy looked at the bat and the wire and the four men who had clearly not come to place a drink order. He looked at his delivery truck and the two men unloading it, both of whom were unarmed because Tommy did not allow guns in his establishment. He looked at the mouth of the alley where a fifth man was now standing, a shotgun cradled in his arms.

Who is asking? Tommy said.

The lead man swung the bat. It hit Tommy in the ribs with a sound like a wet branch snapping, and Tommy went down onto the greasy cobblestones, the breath driven out of him in a single explosive gasp. The other three men went to work on the delivery truck, smashing the bottles one by one, sending rivers of raw gin cascading onto the alley floor. The smell was so strong it made Tommy's eyes water, or maybe that was the pain.

That is for Blue River Blues, the lead man said. Tell the kid he plays it again, we take his fingers. You let him play it again, we take your wife. You understand?

Tommy understood. He lay on the cobblestones, clutching his ribs, watching the gin soak into the cracks between the stones, and he understood perfectly.

The problem was that understanding did not matter. Understanding did not change the chain reaction that had already begun.

The smashed shipment had belonged to Tony Accardo, a Capone affiliate who ran the South Side's liquor distribution and who did not appreciate having his property destroyed by North Side thugs. When Tommy told him what had happened, when he explained about the song and the kid and the territorial dispute, Accardo listened with a face like carved granite and then said three words that Tommy would remember for the rest of his life.

This means war.

Three days later, a Moran card game on Division Street was hit by two men with Tommy guns. Six dead, including Frankie Scales, the lieutenant with the ulcer and the marital problems. The North Side retaliated by firebombing a Capone warehouse on Cicero Avenue. Capone's men responded by gunning down a Moran bookie in broad daylight on Michigan Avenue. The Moran crew answered by killing two Capone soldiers outside a barbershop on Taylor Street. The tally of bodies climbed from six to twelve to twenty, and every number was a direct consequence of a seventeen year old boy playing a cornet in a basement speakeasy on a Thursday night in October.

Tommy watched it all from the sidelines, tending his bar and counting his losses and sleeping with a revolver under his pillow. The customers stopped coming. The musicians found other gigs. The Chinese laundry next door boarded up its windows and the pawnshop owner started carrying a pistol in his belt. The Green Door, which had been a modest success for seven years, was dying by inches, strangled not by the violence itself but by the fear of violence, the creeping certainty that the war would eventually find its way to this quiet basement on Wabash Avenue.

Mae tried to hold things together. She came to work every day, even when she was so nauseated she could barely stand. She kept the books and ordered the supplies and paid off the cops, and at night she lay in bed beside Tommy and listened to the distant gunfire that had become the city's lullaby, and she did not complain. She had never complained about anything in her life. That was what Tommy loved about her, and it was also what frightened him most.

We should leave, he said one night in November, the words coming out before he could stop them. Chicago, the bar, all of it. We should take what we have and go somewhere where nobody knows our names.

Mae turned her head on the pillow and looked at him. Her green eyes were tired but steady. And do what? she said. You are a bootlegger, Tommy. You do not know how to do anything else. You do not know how to be anyone else. And I do not want you to be anyone else. I want you to be the man I married.

The man I married, Tommy thought. The man who had once walked fifteen miles through a blizzard to bring her medicine when she had pneumonia. The man who had worked double shifts at the stockyards for six months to buy her a wedding ring. The man who had never lost a fight, never backed down from a threat, never let anyone push him around. Before the Green Door, before the gin and the payoffs and the careful politics of survival, he had been that man. He had been harder and prouder and more reckless. He had been someone who would have faced down the North Side crew with nothing but his fists and his fury.

But that man had never had a wife who was pregnant with his child. That man had never had something to lose.

The war continued. December brought snow and more bodies. January brought ice so thick on the lake that the rum runners had to haul their boats across the frozen surface by hand. February brought the worst cold snap in a decade, temperatures so low that the glass in the streetlamps cracked and the elevated trains ran two hours late and men were found frozen to death in doorways. And through it all, the chain reaction from that single catalytic moment continued to spread, a domino fall that had started with a song and had ended with a city at war with itself.

Tommy kept the Green Door open because he did not know what else to do. The customers were few now, mostly old regulars who were too stubborn or too broken to care about the danger. Leon Washington had disappeared after the warning. Tommy had told him to leave town, had given him fifty dollars and a train ticket to St. Louis, had watched the kid board the Illinois Central with his battered cornet case and his long musician's fingers and his face full of fear and confusion. He never learned whether Leon had made it. He never learned whether the song had been original or stolen. In the end, it did not matter.

What mattered was Mae.

It happened on a Tuesday night in March. The weather had finally broken, the ice on the lake cracking and groaning like a living thing. Tommy was behind the bar, wiping down the scarred oak surface for the hundredth time that day, when the door opened and three men walked in. They were not customers. Customers did not wear their hats pulled low and their coat collars turned up and their right hands buried in their pockets.

Mae was standing at the end of the bar, counting the till. She looked up when the door opened, and Tommy saw her face go still, and he knew.

Get down, he said, but the word had barely left his mouth when the first man pulled his hand from his pocket and Tommy saw the gleam of blued steel and the black hole of the muzzle. The shot was deafening in the narrow room, a thunderclap that shook the bottles on their shelves. The mirror behind the bar shattered, raining shards of silvered glass onto Tommy's head and shoulders.

Mae, Tommy screamed, but Mae was already on the floor, and there was blood spreading across the front of her dress like a rose blooming in fast motion, and Tommy was vaulting over the bar and running toward her and the three men were backing out the door, their work done, their message sent, their faces blank and professional.

The hospital was six blocks away. Tommy carried Mae the whole distance, running through the frozen streets with his wife in his arms, her blood soaking through his shirt and freezing in the bitter wind. The doctors did what they could. They were good doctors, the best that St. Luke's had to offer, but there was only so much that medicine could do against a bullet that had torn through a lung and nicked the great artery that ran alongside the heart.

She died at four in the morning, with Tommy holding her hand and the monitors beeping their slow descent into silence. She died without regaining consciousness. She died without knowing that the baby, their baby, their first and only child, had died hours before her, too small and too fragile to survive the trauma that had ripped through its mother's body.

Tommy Callahan sat in the hospital room for a long time after they took Mae away. The nurses came and went. A doctor asked him questions he did not answer. A policeman wrote things in a notebook and then left when it became clear that Tommy was not going to say anything. The sun came up over Lake Michigan, pale and cold and indifferent.

At eight in the morning, Tommy stood up and walked out of the hospital. He walked through the waking city, past the newsboys hawking the morning editions and the shopkeepers opening their doors and the streetcars clanging their bells. He walked past the Green Door, which was cordoned off with police tape, its windows dark. He walked south, toward the stockyards, toward the neighborhood where he had grown up, toward the tenement where Mae had lived when she was a girl with red hair and green eyes and a laugh that could light up a room.

He walked until he reached the bank of the Chicago River, where the water ran brown and slow between its concrete walls, carrying the filth of the city toward the lake. He stood there for a long time, watching the water, thinking about a song that had started all of this, a seventeen year old kid and his battered cornet and a melody so beautiful that it had destroyed everything Tommy loved.

The man I married, Mae had said. I want you to be the man I married.

Tommy Callahan had been that man once. He had been hard and proud and fierce. He had never lost a fight. But the fight he was about to start was not one he could win. It was a fight against an entire criminal empire, against the North Side and the South Side and every gangster in between. It was suicide, and he knew it, and he did not care. The chain reaction that had begun with a single note on a cornet had reached its terminal velocity. There was nothing left to lose. There was nothing left to protect. There was only the fire.

He turned away from the river and began walking back toward the city. He did not know how many hours he had left, or how many men he would take with him before he fell. He did not know whether anyone would remember his name or Mae's name or the song that had started it all. He knew only that he was going to walk into the North Side headquarters with a gun in each hand and a heart full of grief that had nowhere else to go, and he was going to finish what a seventeen year old boy had unknowingly begun on a Thursday night in October.

The city hummed around him, indifferent and eternal. The elevated train rumbled overhead. The wind off the lake cut through his coat like a blade. Somewhere, a radio played jazz, the notes drifting through an open window and fading into the grey morning air. Tommy Callahan walked toward his end, and the music played on.