Border crossing involves a change of state; the crossing of a barrier between two nations. The nature and meaning of this barrier are very specific to its location and the different characters of the nations involved. Borders can mark dramatic transitions between different cultures, political systems, economies, and levels of freedom. They range from open and undefended to closed and heavily militarized.
In many places around the world, borders are under stress and evolving to respond to war, migration pressures, the ebb and flow of political alliances, the creation and removal of trade barriers, the threat of terrorism, the movement of illegal drugs and weapons, and the spread of disease.
