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20th century

On February 7, 1904, the Great Baltimore Fire destroyed over 1,500 buildings in 30 hours, leaving more than 70 blocks of the downtown area burned to the ground. Damages were estimated at $150 million in 1904 dollars. As the city rebuilt during the next two years, lessons learned from the fire led to improvements in firefighting equipment standards.

Baltimore lawyer Milton Dashiell advocated for an ordinance to bar African-Americans from moving into the Eutaw Place neighborhood in northwest Baltimore. He proposed to recognize majority white residential blocks and majority black residential blocks and to prevent people from moving into housing on such blocks where they would be a minority. The Baltimore Council passed the ordinance, and it became law on December 20, 1910, with Democratic Mayor J. Barry Mahool 's signature. The Baltimore segregation ordinance was the first of its kind in the United States. Many other southern cities followed with their own segregation ordinances, though the US Supreme Court ruled against them in Buchanan v. Warley (1917).

The city grew in area by annexing new suburbs from the surrounding counties through 1918, when the city acquired portions of Baltimore County and Anne Arundel County. A state constitutional amendment, approved in 1948, required a special vote of the citizens in any proposed annexation area, effectively preventing any future expansion of the city's boundaries. Streetcars enabled the development of distant neighborhoods areas such as Edmonson Village whose residents could easily commute to work downtown.

Driven by migration from the deep South and by white suburbanization, the relative size of the city's black population grew from 23.8% in 1950 to 46.4% in 1970. Encouraged by real estate blockbusting techniques, recently settled white areas rapidly became all-black neighborhoods, in a rapid process which was nearly total by 1970.

The Baltimore riot of 1968, coinciding with uprisings in other cities, followed the assassination of Martin Luther King Jr. on April 4, 1968. Public order was not restored until April 12, 1968. The Baltimore uprising cost the city an estimated $10 million (US$ 88 million in 2024). A total of 12,000 Maryland National Guard and federal troops were ordered into the city. The city experienced challenges again in 1974 when teachers, municipal workers, and police officers conducted strikes.

By the beginning of the 1970s, Baltimore's downtown area, known as the Inner Harbor, had been neglected and was occupied by a collection of abandoned warehouses. The nickname "Charm City" came from a 1975 meeting of advertisers seeking to improve the city's reputation. Efforts to redevelop the area started with the construction of the Maryland Science Center, which opened in 1976, the Baltimore World Trade Center (1977), and the Baltimore Convention Center (1979). Harborplace, an urban retail and restaurant complex, opened on the waterfront in 1980, followed by the National Aquarium, Maryland's largest tourist destination, and the Baltimore Museum of Industry in 1981. In 1995, the city opened the American Visionary Art Museum on Federal Hill. During the epidemic of HIV/AIDS in the United States, Baltimore City Health Department official Robert Mehl persuaded the city's mayor to form a committee to address food problems. The Baltimore-based charity Moveable Feast grew out of this initiative in 1990.

In 1992, the Baltimore Orioles baseball team moved from Memorial Stadium to Oriole Park at Camden Yards, located downtown near the harbor. Pope John Paul II held an open-air mass at Camden Yards during his papal visit to the United States in October 1995. Three years later the Baltimore Ravens football team moved into M&T Bank Stadium next to Camden Yards.

Baltimore has had a high homicide rate for several decades, peaking in 1993, and again in 2015. These deaths have taken an especially severe toll within the black community. Following the death of Freddie Gray in April 2015, the city experienced major protests and international media attention, as well as a clash between local youth and police that resulted in a state of emergency declaration and a curfew.