From America Magazine, 2/16/16:  
"In truth, Catholic teaching, as articulated by several popes through the years, is remarkably clear on national sovereignty and borders. Pius XII, the author of “Exsul Familia”—known as the Magna Carta on Catholic teaching on migration—wrote to U.S. bishops in 1948 that while national sovereignty “must be respected,” migrants should be accepted as long as “the public wealth, considered very carefully, does not forbid this.” Saint John XXIII, in his encyclical, “Pacem in Terris” ("Peace on Earth"), taught that nation-states have an obligation to the “universal common good” that requires them to welcome newcomers: “When there are just reasons in favor of it, he [every human being] must be permitted to emigrate to other countries to take up residence there.” (No. 25).

"When there are just reasons..."  That's why I asked, "Aren't there places in their own countries where can live in peace and happiness?"  Because if there are,... And if not,...