Short answers, pulled from the story.
Brooklyn had a population of 2,736,074 in the 2020 United States census, making it the most populous of New York City's five boroughs and the most populous county in New York State. If it had remained an independent city, it would rank as the fourth most populous city in the United States, after the rest of New York City, Los Angeles, and Chicago.
Brooklyn's name derives from the Dutch town of Breukelen in the Netherlands. The word combines broeck, meaning bog or marshland, and lede, meaning a small dug water stream in peat areas. The American settlement of Breuckelen was established in 1646, and the name first appeared in print in 1663.
Brooklyn was consolidated into New York City on the 1st of January 1898, following a 1894 vote in which residents of Brooklyn and surrounding counties approved the merger by a slight majority. Many newspapers of the day called the event the "Great Mistake of 1898," reflecting significant opposition from Brooklyn residents who valued the borough's independent identity.
The Battle of Brooklyn, also known as the Battle of Long Island, was fought on the 27th of August 1776. It was the first major engagement of the American Revolutionary War after independence was declared and the largest battle of the entire conflict. Although British forces defeated George Washington's Continental Army, Washington's overnight evacuation of all his troops and supplies across the East River is now regarded by historians as one of his most brilliant military achievements.
The second recorded game of baseball was played near what is now Fort Greene Park on the 24th of October 1845. Brooklyn teams dominated organized baseball from the mid-1850s through the Civil War, and according to the historical record, the first fastball, first batting average, first triple play, first professional baseball player, and the first eight pennant winners all originated in or from Brooklyn.
The Brooklyn Dodgers won their only World Series in 1955, defeating the New York Yankees. Two years later, owner Walter O'Malley relocated the team to Los Angeles as part of a realignment of Major League Baseball in 1957. O'Malley is still vilified in Brooklyn for the move.