A more thorough explanation:

The doctrine of contagion is a discredited belief in international law that justifies the invasion and overthrow of a neighboring state's government due to the spread of revolution or abhorrent practices. This doctrine was used by the Holy Alliance in Europe from 1815 to 1848 to invade countries where revolutions were brewing.

For example, if a neighboring country is experiencing a revolution, the doctrine of contagion would argue that the revolution could spread to other countries and threaten their national security. Therefore, the neighboring country's government could be overthrown to prevent the spread of revolution.

This doctrine was used by the Holy Alliance to invade countries like Spain and Italy, where revolutions were taking place, to prevent the spread of revolution to other countries.