As the world filled up with nation-states, and empty spaces filled up with people, restriction triumphed over free movement. Controls on immigration became widespread after World War I. All countries developed population policies.
世界由民族国家组成，空白空间由人填满，因此限制战胜了自由流动。对移民的控制在一战后变得日益普遍。所有国家都制定了人口政策。