Why Does the Pentagon Have Five Sides?

The plot of land that the Pentagon was originally planned to be built on was bound on five sides due to roads and other divisions, so the Pentagon was designed to conform to these restrictions. When the build site was moved, the Pentagon kept its original shape.

Objections concerning the sensitivity of building a military complex so close to the Arlington National Cemetery were the cause for this change in building location. With no time to start a new design process, they decided that it would be best to keep the Pentagon's original shape, although its five sides were smoothed and straightened to form the more regular pentagon that we see today.