Richard K. Betts, Adjunct Senior Fellow for National Security Studies
The way in which contiguous borders became less important is not new, but came with the advent of air power, which enabled countries to attack targets in an enemy's interior without conquering the enemy's defending army. The effectiveness of air power has grown in the past few decades, and for certain types of limited strikes the technology of drones has provided new options.
The changes are ones of degree, however, and air power has not yet made armies obsolete as early air power enthusiasts predicted. Air attack alone has almost never forced an enemy surrender. The one exception so seems to be the 1999 war over Kosovo, when Belgrade surrendered after seventy-eight days of bombing alone. Even in that case, however, some believe that the result was affected by Serbia's fear that NATO was about to move toward an invasion on the ground.
For the most part borders and proximity remain very relevant militarily. First, political disputes over borders—which side owns the territory in question—are often what cause the conflict in the first place. Second, in interstate wars, the determining military factor usually remains the clash of armies on the ground. It is much easier logistically to deploy large ground forces across a contiguous border than to get them into action far away, where transport, supply, and sustainment are extremely difficult. Third, in internal or civil wars, rebel forces often depend on support and supplies from sanctuaries in an adjoining country.