The traditional concept of sovereignty is largely independent of democratic legitimacy and completely indifferent to any obligation towards non-national citizens. But can this traditional concept meet the normative expectations of a post-traditional understanding of political authority as well as the challenges of an ever more interconnected world? In order to respond to this question, the Article analyzes the conceptual presuppositions that lie at the basis of the notion of “sovereignty,” first regarding its sources, and second regarding the ideas of rationality that are applied when sovereign actors operate. As far as the sources of sovereignty are concerned, it is argued that both of them - the “ascending” and the “descending” - although decisive for determining the quality of the legitimacy of political power, have little influence on a positive attitude of sovereigns towards aliens’ interests. To clarify the conditions for an opening of sovereign powers to solidarity, an assessment of the rationalities which are implemented when a sovereign puts actions into effect is therefore required. Yet most rationality concepts - or uses of practical reason - prove to be negative or at least useless when it comes to the question of supporting solidarity: the gamut ranges from open hostility towards the idea of taking the interests of aliens into account, through substantial indifference, to a positive approach which presupposes, however, non-provable metaphysical assumptions or an individual mindset with no pretension of issuing norms of general validity. Only the communicative conception of reason meets the criteria for a convincing justification of solidarity towards aliens as an obligation. The author therefore concludes that only an “ascending” sovereignty based on a communicative understanding of rationality can be considered fully legitimate insofar as the sovereign power, in this case, first originates from the will of the citizens and, second, is morally, politically and legally obliged to a solidaristic attitude towards the justified interests of non-citizens.