A global event, such as a natural catastrophe, would do it. Often times, in highly stressful moments people otherwise unlikely to work together do so.
Non catastrophic moments that would carry the same power might be possible by a great discovery or revelation that touches each person in a way profound enough to distract them from the normal rat-race of the day to day. How such a thing could manifest in a way that does not provoke the engagement of armies and other forces designed and created to instill public order is another good question to be asking.
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