Merger of the City of New York

January 1, 1898:

 New York City annexes land from surrounding counties, creating the City of Greater New York. The four initial boroughs, Manhattan, Brooklyn, Queens, and The Bronx, are joined on January 25 by Staten Island to create the modern city of five boroughs.
New York's commuter zone had only reached into the middle of Kings County, the Annexed District, and western Queens, so for decades the consolidated municipal government was in charge of mile upon mile of farmland and dusty country roads.

What amazes me is that Brooklyn was the fourth largest city in the United States in 1890, behind New York City (Manhattan), Chicago and Philadelphia (it was third largest in 1880, before the rise of Chicago). 

Also, New York City contains 8.1 million people in an area 2/3 the area of my county.