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U.S. President Donald Trump Friday declared a "national emergency" to enable him to fund hundreds of miles of wall on the southern border. Trump says the wall would prevent migrants from crossing into the United States illegally from Mexico. What's the fundamental reason for Trump to build a wall between the U.S. and Mexico? Why is it so urgent? What if the next president calls it off? CGTN talks to Li Yong, a senior fellow of the China Association of International Trade to find out more.