Pope Francis said in a letter Monday addressed to a Vatican conference in Mexico City that the tens of thousands of undocumented Central American children crossing the border in the U.S. should be "welcomed and protected." The pontiff said that immigration has been a hallmark of modern society. "Many people forced to emigrate suffer, and often, die tragically," Francis said. "Many of their rights are violated. They are obliged to separate from their families and, unfortunately, continue to be the subject of racist and xenophobic attitudes." He highlighted the crisis at the U.S. border and said the world must promote development in the home countries from where the children emigrate.