Lori Lightfoot, a former police board president, and Toni Preckwinkle, president of the Cook County Board of Commissioners, finished first and second in Tuesday night’s election and are heading to an April run-off. Whoever wins, it will be historic. It will mark the end to the most crowded, least predictable mayor’s race in the history of America’s third-largest city. And it was catalyzed by a tragedy five years ago, and the protest movement that followed. In Chicago, the next mayor is usually pre-ordained by the so-called “Chicago machine” — a Democratic party-run system of favoritism and financial ties that make most elections in the city all but a foregone conclusion. This infamous machine helped keep Chicago’s most famous mayor, Richard J. Daley, in office for two decades. It helped Daley’s son, Richard M. Daley, get reelected five times. And it helped Mayor Rahm Emanuel get elected twice. But in 2014, a white police officer shot and killed a black teenager. This was, unfortunately, a familiar story in Chicago — until a protest movement forced change. The cop who killed Laquan McDonald was convicted of murder and the state’s attorney was defeated at the ballot box. Many Chicagoans blame bias in the system for McDonald’s death and police brutality against black men and women more broadly. The movement his death sparked hasn’t stopped; the public and government have spent the five years since grappling with how to reform government and its institutions. Eventually, this public discontent forced Emanuel to drop out of the mayoral race. That opened the floodgates for a 14-candidate ballot, the largest number in the city’s history. For the first time in decades, the machine didn’t have control of this race. So no one really knew who was going to win. The protest movement — the young people who helped create this moment in Chicago — spent the last several months trying to transfer momentum from the street to the ballot box.