By 1860, had become the largest city as its population surpassed one million. boston philadelphia baltimore new orleans new york

New York--In 1860, New York was the largest city in the US. 

Large numbers of immigrants and growing trade led to New York being the largest US city. The city would continue to grow after the Civil War to become the hub to immigrants and the center of US trade. 

Contributing factors  leading  to the harlem renaissance were the great migration of african americans to northern cities, which concentratedambitious  people in places where they could encourage each  other, and the first world war, which had created new industrial work opportunities for tens of thousands of people.