Legal Definitions - contagion

Contagion is a doctrine that has been discredited in international law. It suggests that if a neighboring country is experiencing a revolution or engaging in practices that are considered abhorrent, then it justifies an invasion of that country and the overthrow of its government in the name of national security.

The Holy Alliance, which existed from 1815 to 1848 in Europe, used this doctrine to invade countries where revolutions were brewing. However, this doctrine is no longer considered valid in international law.

For example, if Country A is experiencing a revolution, and Country B believes that this revolution could spread to its own country, then Country B may use the doctrine of contagion to justify an invasion of Country A. However, this is no longer considered a valid reason for invasion.