43 Whenever there exists, or there is reason to fear, an unjust oppression of the people on the one hand, or a deprivation of the liberty of the Church on the other, it is lawful to seek for such a change of government as will bring about due liberty of action. In such case, an excessive and vicious liberty is not sought, but only some relief, for the common welfare, in order that, while license for evil is allowed by the State, the power of doing good may not be hindered.
Source: Libertas (Vatican.va)