Confederate General Robert E. Lee (1807 - 1870) was married to the great-granddaughter of George Washington, hero of the Revolutionary War, and the first president of the United States. Her name was Mary Anna Randolph Custis Lee (1807 - 1873).
"Arlington Heights, in the spring of 1861, was a 1,100-acre plantation overlooking the Potomac River and the city of Washington, D.C.Then came that damn war which claimed 700,000 casualties, obliterated the execrable Dred Scott Decision, freed Negro slaves from their southern Democrat slave masters, and eventually led to the assassination of Pres. Lincoln by the drunken coward (or vengeful hero, depending on your perspective) John Wilkes Booth.
Mrs. Lee inherited Arlington in 1857 upon the death of her father George Washington Parks Custis."
Lee's former home was chosen by Secretary of War Edwin M. Stanton as the final resting place for many of the war dead - both Union & Confederate.
However, it's erroneously called 'The American Civil War.' Why erroneously? By definition, a civil war is when two or more factions fight for control of the same government. The Confederacy wished to secede from the Union, and form its own nation - not fight for control the Union government.
It's more accurately called 'The War of Secession.' (And no, 'the war between the states' ain't a thing)
The history of Arlington National Cemetery