Widespread belief in economic liberalism in the second half of the nineteenth century, combined with the development of safer, faster, and cheaper transportation, paved the way for huge migration to occur. Between 1850 and 1914, 55 million people departed Europe, with the vast majority heading to the Americas during what Hatton and Williamson term "the age of mass migration". According to McKeown, something similar in scale and duration took place at approximately the same time – albeit

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... for slightly longer – involving Indians and southern Chinese moving to Southeast Asia and people from north-eastern Asia and Russia to North Asia. However, "the booming of the guns of August 1914 brought to a sudden close the era during which foreigners were relatively free to traverse borders", according to John Torpey. States in Europe and North America, in particular, reintroduced passport controls with vigour during World War I and instead of lifting these bellicose measures at the end of the conflict, they generally reinforced them. The United States led the way in introducing such restrictions. Following on from the imposition of the 1917 Literacy Act came the 1921 and 1924 US Immigration Acts, which limited arrivals by introducing quotas for countries. The development in much of Europe of the modern welfare state in the late nineteenth and early twentieth century simultaneously gave rise to more restrictive immigration policies in Europe, thereby leading to an even greater distinction between citizens and non-citizens.