Upon the outbreak of the great European conflict in 1914 the flow of immigration subsided quickly and while the war lasted it practically ceased altogether. This was due in part to the reluctance of European countries to let their people leave, and in part due to the lack of shipping, since all available vessels were engaged in carrying troops, munitions, and supplies. Immediately after the close of hostilities, however, the exodus from Europe recommenced, and in the year 1920 the figures of immigration once more rose into the hundreds of thousands. In spite of the strict regulations imposed by the United States government, enormous numbers of people in Poland, Italy, Hungary, and various other countries began applying for permission to migrate. The American consular offices in Europe were besieged by long lines of people waiting all day for passports to the Promised Land. In the face of this threatened flood Congress felt impelled to place all immigration under strict limitation, as will be explained a few paragraphs later. During the hundred years from 1820 to 1920 it is estimated that more than thirty million immigrants reached the United States, which is the largest migration in history.[12] It is true that considerable numbers of these immigrants eventually drifted back to their native lands but the larger portion of them remained permanently here. [Sidenote: The various factors which have caused Europeans to leave their own countries.] =Causes of Immigration.=—The causes of immigration to this country are manifold. First in importance among them is the economic attractiveness of the United States. Cheap land, equal opportunities, the ease with which any industrious man or woman can make a living—these are the magnets which drew millions of people across the Atlantic during the nineteenth century. But there were other causes, namely, the existence in various parts of Europe of conditions which encouraged the people to emigrate. Religious persecution was one of the first things that drove men or women to America. It brought the Pilgrims to Massachusetts in the early days, the Quakers to Pennsylvania, and the English Catholics to Maryland. Political oppression in the middle years of the nineteenth century sent several million Germans to the United States, particularly to the Middle West. Oppression, both religious and political, has been instrumental in driving the Jews across the seas. Overcrowding in both the cities and the rural regions of Europe and the consequent inability of the people to obtain lands of their own, has been responsible for much of the drift to America. The exhaustion of the soil in Ireland, the crop failures and consequent famines there, have led to the migrations of the Irish, although dissatisfaction with political conditions has also been a factor in this case. To a considerable extent, moreover, immigration has been assisted by those aliens who were already settled in this country.