its War on Terrorism (See Shantz 2010). One American writer has said that the US-Mexico border is probably "the world's longest boundary between a First World and Third World country". Historic borders such as the Great Wall of China, the Maginot Line, and Hadrian's Wall have played a great many roles and been marked in different ways. While the stone walls, the Great Wall of China and the Roman Hadrian's Wall in Britain had military functions, the entirety of the Roman borders were very porous, which encouraged Roman economic activity with neighbors. On the other hand, a border like the Maginot Line was entirely military and was meant to prevent any access in what was to be World War II to France by its neighbor, Germany; Germany ended up going around the Maginot Line through Belgium just as it had done in World War I. Border conflict Border conflicts or the potential of such are the reason why many borders feature fortifications and zoning like no man's lands, demilitarized zones, demarcation lines and buffer zones. Examples of border conflicts include skirmishes and wars, such as 38th Parallel (between North and South Korea), Western Sahara conflict, Kashmir region (between India and Pakistan), etc. Border construction and crossings Borders have sometimes been significantly shaped by physical border constructions and openings. From border crossings, over border markers to border barriers these constructions fulfill many different functions. Such as also providing crossover. Crossings and transition Even the most fortified borders reserve specific places to allow crossing. The many forms of borders have different ways of enabling and controlling passage. Movement and permeability Borders can have an significant impact on and function for movement. It can enable and stop movement, across as well as along borders. The permeability of borders depends on its construction, availability of crossings, regulation and types or scope of activity. The permeability can vary, borders can be barriers for humans, but also for animal migration or types of pollution. Overlap and cooperation Borders facilitate or block hybrids like border overlap and cooperation beyond mere encounter and exchange. Cross-border regions Macro-regional integration initiatives, such as the European Union and NAFTA, have spurred the establishment of cross-border regions. These are initiatives driven by local or regional authorities, aimed at dealing with local border-transcending problems such as transport and environmental degradation. Many cross-border regions are also active in encouraging intercultural communication and dialogue as