202 Individual political communities may indeed enjoy a high degree of culture and civilization. They may have a large and industrious population, an advanced economic structure, great natural resources and extensive territories. Yet, even so, in isolation from the rest of the world they are quite incapable of finding an adequate solution to their major problems. The nations, therefore, must work with each other for their mutual development and perfection. They can help themselves only in so far as they succeed in helping one another. That is why international understanding and co-operation are so necessary.
Source: Mater et Magistra (Vatican.va)