Brian: In the 1850's, Congress created the Nevada Territory, carving it from Utah. Congress handed over more land when Nevada became a state in 1864. But there was one thing Nevada lacked - a water supply.
Mark: That whole triangle at the bottom of Nevada had been part of what was then the Arizona Territory - since it was just territory, Congress could still take it. And it gave Nevada, among other things, access to a piece of the Colorado River.
Brian: Why did Congress take Arizona's land and give it to Nevada? Well, because Arizona had sided with the South during the Civil War. It was payback time.