This chapter investigates the Allied occupation of France, especially the way this occupation was managed politically between 1815 and 1818, as an instance of the new European security system and corresponding culture at work. The preparations for this occupation, mainly through the deliberations and activities of the Allied Council of Ambassadors, provide a perfect case study to show how contentious and conflict-ridden this Allied cooperation was (on matters of finance, French politics and military matters), but also how it moved towards a more collective, consensual method of decision making in a highly volatile environment. This – until the present not fully contextualised and researched – Allied Council of Ambassadors responsible for executing the occupation was arguably the first instance where Europe’s system of collective security was inaugurated on the ground, albeit haphazardly and contested, between 1815 and 1818.