How do different-sized powers establish order among themselves? And how do larger powers compel smaller and weaker ones to do their bidding? Though abstract, these two questions have concrete applications in the real world and are among the oldest in international relations. Differential power relationships have been around since the first polities, later called states, came into existence, and over the course of history, various systems have arisen to manage them. Among the best-known examples from the classical world are the Greek alliances, also called leagues, which at times kept the peace, but also, when that failed, made wars far more costly than they otherwise might have been. Alliances and alliance-style relations, it turns out, are double-edged swords. As the political scientist Robert Putnam once observed in a seminal article, they also involve “two-level games,” whereby domestic factions compete with one another to dominate foreign affairs. Alcibiades and his generation knew a thing or two about playing one party off against the other in Athens’ client states.
Traditionally, a league -- such as the one headed by Athens -- becomes an empire when the so-called hegemon acts by force to prevent a smaller polity from seceding or to compel some other action, such as greater tribute. But in theory, at least, the glue for such arrangements is usually voluntary. Smaller states generally want to ally with a big power for obvious reasons of self-preservation, even if the relationship entails high costs. This so-called empire by invitation has also been characterized by the historian Geir Lundestad, with reference to the Atlantic Alliance, as an empire by integration, meaning that such arrangements assimilate patterns of authority and norms of state behavior. The notion is a tempting one, but such a flexible definition of empire and imperialism ultimately raises questions about the exercise of hegemonic rule. After all, from the point of view of the inviting power, such an invitation to integrate is not the same thing as a demand to do so. But what about from the perspective of the invited power? Indeed, sometimes the one doing the inviting is not the stronger power.
Readers of Thucydides will recall the Melian dialogue, which follows along these lines: The Athenians tell the Melians to submit; the Melians resist by pointing out that Athenian power would lose a good deal of legitimacy and attractiveness if it were seen to be so coercive. Such exchanges, like the one between the Dutch and the Spanish in the 17th century or between the Americans and the British in the 18th, tend to be resolved by the sword one way or another.