Detroit was the largest city of the state of Michigan in the United States of America, on the North American continent of Earth. Detroit played a major role in Earth's industrialization, and was a center of automobile manufacture for most of the 20th century.
In the mid-20th century, people, even those in rural areas, could hear about what was happening in this city on the radio. Though people might feel they were far from events, radio brought them in closer. (TNG: "The Big Goodbye")
In 2004 three Xindi-Reptilians arrived from the year 2153, having been covertly transported through time by the Sphere-Builders. There, they attempted to construct a bio-weapon to use against the Humans. There, they engaged Loomis, an employee at a Detroit blood donors clinic, to obtain samples of all Human blood types in order to perfect the weapon. However, they were eventually stopped by Jonathan Archer and T'Pol of Enterprise NX-01, who had been transported there by temporal agent Daniels and Loomis was arrested by the Detroit Police.
In 2153, the only map of early 21st century Detroit that T'Pol managed to find in the Enterprise's database was from 2002. (ENT: "Carpenter Street") T'Pol's visit to Detroit tempered her skepticism on the concept of time travel. (ENT: "Azati Prime")