1) New York City grew rapidly in the late 19th century due to economic and population growth, becoming the most populous city in the US. 2) Jacob Riis documented the squalid living conditions of immigrants in tenement housing in his landmark book "How the Other Half Lives," bringing attention to reforms needed. 3) In the 1920s, New York City became the focal point of American culture as a cosmopolitan melting pot, though the Great Depression soon followed.