According to the International Relations theory known as Realism, interstate interactions, whether ancient or modern, are motivated by the pursuit of hegemony of individual states, which act as monolithic groups in articulating their foreign policy decisions. The application of Realism to the study of Spartan foreign policy in the third century BC shows the validity of this theory in explaining certain aspects of ancient interstate interactions, as illustrated by the two alliances discussed in this article. The first, earlier alliance, between (...) King of Macedonia Antigonos Gonatas and Sparta and some Peloponnesian cities, was made in response to the threat posed by King Pyrrhus of Epirus who was set on conquering them. The later alliance, between Sparta and other states, was made to stop the ambitions of King Antigonos Gonatas who controlled many cities of the Peloponnese and sought to conquer the whole of Greece. In forging such alliances, according to the Realist view, the states acted as large monolithic groups, yet a broader assessment of the available evidence shows the presence of smaller, subtler networks of individuals, which were paramount in articulating important foreign policy decisions. The comparison of Spartan decision making with the activation in 2009 of the EU’s Solidarity Clause, included in Article 222 of The Treaty on the Functioning of the European Union (TFEU), indicates that the activation of this clause too required the consensus of several European governing bodies. By bringing to light the critical role played by smaller networks of power in foreign policy decision making in the ancient and the current-day EU alliances, this article exposes both the merits and the limits of the Realist international relations theory. (shrink)