I don't see why we haven't moved our capital to the middle of the United States. Right now our HQ is a stones throw away from the Atlantic Ocean, if someone wanted to attack they would have a tough time getting through all of our defenses.
Imagine that we had it in the middle of our country, not only do we have the Atlantic and Pacific Ocean around us but if we keep up good relations with Canada and everything under the border, potential attacks would have to break though serveral hundreds of miles of land defense as well. This makes any kind of external attack damn near impossible.