The research examines how the criteria for permissible military intervention by invitation as developed in international law doctrine are currently implemented by States, as well as how this impacts the prohibition of the use of force against the territorial integrity or political independence of States. Controversies concern in particular the determination of the authority entitled to extend the invitation, as recently illustrated by the Russian claim that its military intervention in the Crimea in 2014 was based on the invitation of (former) President Yanukovych. Does the inviting authority need to enjoy democratic legitimacy, or is effective control (still) the point of departure for determining the legitimate government of a State? Furthermore, it remains highly contentious whether an invitation for forcible intervention may be extended during a civil war. Once recognized, how much discretion does the incumbent government enjoy when inviting military assistance from foreign governments? Do incumbent governments retain the right to military assistance even in situations of civil war and while exercising limited control over the territory? Or would an intervention under these circumstances violate the right to self-determination and, in turn, the prohibition of the use of force? The analyses focuses on modern State practice in particular in Africa – where most of the contemporary invitations for military assistance occur – and compares it to recent developments in other regions, such as Afghanistan and Iraq.