manhattan s little italy, lower east side, circa 1900 the construction of new york city subway, opened in 1904, helped bind new city together, did additional bridges brooklyn. in 1920s manhattan experienced large arrivals of african-americans part of great migration southern united states, , harlem renaissance, part of larger boom time in prohibition era included new skyscrapers competing skyline. new york city became populous city in world in 1925, overtaking london, had reigned century. manhattan s majority white ethnic group declined 98.7% in 1900 58.3% 1990. on march 25, 1911, triangle shirtwaist factory fire in greenwich village killed 146 garment workers. disaster led overhauls of city s fire department, building codes, , workplace regulations. the period between world wars saw election of reformist mayor fiorello la guardia , fall of tammany hall after 80 years of political dominance. city s demographics stabilized, labor unionization brought new protections , affluence working ...