A month after a mass shooting left 17 dead at Marjory Stoneman Douglas High School in Parkland, Fla., thousands crowded into downtown Washington for a protest dubbed the March for Our Lives. Bearing signs and chanting, demonstrators — led by survivors of the Stoneman Douglas massacre — called for action by Congress on gun control in response to the nation’s relentless two-decade stretch of campus shootings. Hundreds of “sibling marches” took place in cities across the United States.