Pope Francis used strong language to describe Europe’s migrant crisis Wednesday, calling it the greatest tragedy since the Second World War.

After his weekly General Audience in Saint Peter’s Square, the Pope offered a special greeting to officials of an Italian Church association for migrants called “Migrantes,” encourage them in their reception and hospitality toward migrants.

Francis underscored the importance of the “integration” of migrants into their host society, as well as the “reciprocal rights and obligations of those who receive and those who are received.”

“Let’s not forget that this contemporary problem of refugees and of migrants is the greatest tragedy since the Second World War,” he said.

Pope Francis has often expressed himself on the question of immigration, encouraging individuals and nations to keep an open heart toward migrants and to treat them according to their dignity as human beings.

Last fall, the pontiff said it is “hypocritical to call oneself a Christian and send away a refugee, or one who needs my help. Jesus taught us who the good Christian is in the parable of the good Samaritan,” he said.

At the same time, Francis has also insisted on the sovereign right of nations to control their borders.

“Yes, every country has the right to control its borders, who comes and who goes,” Francis said in a lengthy interview in January, “and those countries at risk —from terrorism or such things— have even more right to control them more.”

In that same interview, the Pope also underscored the problem of integration, that is, the possibility of successfully assimilating immigrants into the existing culture.

“Where there is no integration,” Francis said, immigrants become “ghettoized,” and do not become part of the culture of their host country.

As an example of this, he said that the perpetrators of “the atrocity in Zaventem,” the Belgian airport, “lived in an immigrant neighborhood, a closed neighborhood.”

After visiting Sweden last November and hearing about some of the problems they were experiencing, Pope Francis offered a nuanced exposition of his views on the question of immigration, along with a distinction between theory and practice.

“So what do I think of those who close their borders?” Francis asked. “I think that in theory no one should close their heart to a refugee, but those who govern must also exercise prudence.”

“They should be very open to receiving them, but they should also calculate how they will be able to settle them, because a refugee must not only be welcomed, but also integrated,” he added.

“And if a country is only able to integrate 20, let’s say, then it should only accept that many. If another is able to do more, let them do more,” he said.

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