> The entire world was relatively borderess prior to 20th century, The Romans spent a lot of effort in trying to keep the various barbarian tribes from migrati

> The entire world was relatively borderess prior to 20th century, The Romans spent a lot of effort in trying to keep the various barbarian tribes from migrating freely into the Roman Empire. In fact, for much of the Roman Empire, the bulk of the Roman Army was stationed on the frontiers such as the Danube and Rhine to keep the German tribes out. One of the big turning points was in 406 when the Rhine froze and removed a natural barrier that had kept a lot of these tribes out before hand. Imperial China also worked hard on keeping the various northern groups from migrating into China, going so far as to build the Great Wall. It seems that the more developed the civilization, the more effort it expends in defining and guarding it's borders.