External cohesion, as far as groups of humans living in a community or a nation are concerned, is a natural survival strategy that occurs when several groups, usually existing in close proximity to each other or embracing a core ideology, are faced with a common threat to their existence which, because of a lack of their own internal resources, they are unable to overcome on their own.
This applies to all types of situations—military conflict, resource deficiencies, natural disasters, and more recently terrorism, international crime, greed in the global financial system, and global warming. This strategy is also used in communities when resources are scarce.
What is unprecedented at this time in our human existence is that the threats to our collective survival are both global and man-made.
Humans have never had to face issues of external stability on this scale ever before.
In order to be a leader of a community or a nation, where there are people with diverse sets of beliefs, you will need to practice values-based leadership and operate with a self-transforming mind.