A nation needs a foreign policy for four reasons. The first is security. The second is that every other nation already has a foreign policy: If you do not assert yours, someone else’s will press upon your borders. These two reasons are physical, and the policies they produce are geographical and spatial: defending the borders, shaping the “sphere of interest.” These are ancient ideas. As Rome needed to control the Alpine passes, Rome had to control the terrain beyond the passes. See the Monroe Doctrine.