Since the earliest times in man's history, people have moved from place to place, looking for better places in which to live, work and raise families. People also migrate to escape natural disasters, famine and persecution.
Sometimes with the arrival of new people, they are able to settle peacefully or they may find themselves in conflict with those people who have already live in the area.
Countries have sometimes created barriers to keep new people out. One of the earliest examples is The Great Wall of China which stretched over 7,300 km. It was built by the Chinese emperor, Shih Huang-ti in the third century B.C. to stop neighbouring tribes from attacking his lands.