It was in danger of being surrounded by enemy states.
DC was actually a slave-state itself for the first few months of the war, although slave-trading had been outlawed there.
Virginia was one of the slave-states of the Upper South that was lukewarm about secession, but voted Confederate after Fort Sumter.
Maryland nearly went the same way, which is why Lincoln suspended Habeas Corpus, in order to jail some of the state's pro-Southern leaders.
Washington then became a strongly fortified city, which seemed impregnable, though the Confederates did manage to get very close to it at certain times.
Remember - from the roof of the White House, you could actually see Robert E. Lee's house.
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The Battle of Fort Stevens was fought in Northwest Washington, D.C. The battle occurred on July 11 and 12, 1864, as a part of the Valley Campaigns of 1864.
The mood was sensual.
The union capital stayed in Washington, DC.
The border states were still in the Union and the capital of the US is Washington, DC. The American Civil war lasted from 1861 to 1865.
Richmond, Virginia and the president was Jefferson Davis this was the capitol and president of the C.S.A. when the south had seeded before the civil war
Maryland was critical to the physical safety and protection of Washington, D.C.