Hours before the start of the Women’s March on Washington on Jan. 21, every Metro car leading to the National Mall was packed so tight that it could take on no more riders. At station after station, the train doors would open, and the passengers would look out on throngs of fellow protesters — women, men, children, babies, the occasional dog — waiting on the platform. As people emerged from underground into the morning air, it was hard to tell where to go, so they found their way by gauging the human density, moving until they reached a spot so full of people that they could no longer move at all. There was an enormous rally happening somewhere in there — activists and celebrities speaking into microphones — but much of the crowd couldn’t see or hear anything except rumbling waves of cheers. By the time they were meant to march, the crowd was so large that it already stretched across the entire route, from the rally site near the Capitol to the Ellipse near the White House.

The signs they carried spoke to any number of issues: immigration, abortion, race, the environment, inequality, the new president. REFUGEES WELCOME, KEEP YOUR LAWS OUT OF MY VAGINA, BLACK LIVES MATTER, SCIENCE IS REAL, FLINT NEEDS CLEAN WATER, NOBODY LIKES YOU. The handmade pink “pussyhats” that many marchers wore — a reference to Donald Trump’s caught-on-tape boasts about grabbing unsuspecting women by the genitals — had been sneered at in the days before the march. They were called corny, girlie, a waste of time. Seen from above, though, on thousands of marchers, their wave of color created a powerful image.

It was, somewhat unexpectedly, one of the largest mass demonstrations in American history. Millions of protesters — estimates range from three to five million — took to the streets of Washington, Los Angeles, New York, Palm Beach, Fla., Boise, Idaho, even Fargo, N.D. Sister demonstrations were held in Thailand, in Malawi, in Antarctica. The energy of almost every group alarmed or incensed by Trump’s election seemed to have poured into a single demonstration. That it happened on the day after his inauguration was not surprising. What was striking was that all these people had come together under the auspices of a march for women.

Just two months earlier, the left did not appear to be a unified front. The polls had barely closed before the infighting began. Some blamed Hillary Clinton for ignoring Wisconsin, or the Democratic National Committee for boxing other candidates out of the primary field. Some blamed identity politics, which made working-class white voters “feel excluded,” according to Prof. Mark Lilla of Columbia. Others blamed white people, particularly the coastal ones who couldn’t get their heartland relatives on their side.