Introduction
What are the limits of a security strategy predicated on relative advantage, and what can be done to mitigate them? Great powers rely on strengths and actively seek to bolster them, particularly vis-à-vis potential adversaries—hence ‘relative advantage’—when considering how to secure themselves. This is seen in the most recent versions of the U.S. National Security Strategy (NSS) and National Defense Strategy (NDS). But relative advantage alone does not guarantee strategic success: there are inherent limits to strategy that relative advantage cannot entirely eliminate. For today’s U.S. strategic thinkers, who are tasked with managing geopolitical competition and achieving national security aims, Thucydides’ account of the Peloponnesian War (431-404 BC) offers invaluable insights into the nature of these limits, their implications for managing great power competition, and how to mitigate them.
Strategic Planning in an Era of Great Power Competition
From the perspective of the United States and its Western allies, today’s globalized world is increasingly complex and unpredictable.[1] The post-World War II rules-based international order is threatened[2] and rivals of the West are gaining ground,[3] with Russia seeking to undermine elements of the system[4] and China attempting to revise it.[5] The question is how to respond – and what a successful strategy looks like.
The NSS and NDS outline the United States’ answer. Their respective aims: “protecting the American people and preserving our way of life, promoting our prosperity, preserving peace through strength, and advancing American influence in the world” and “to compete, deter, and win in this increasingly complex security environment.” The NDS advocates “a more lethal, resilient, and rapidly innovating Joint Force, combined with a robust constellation of allies and partners”[6] while the NSS aims to protect U.S. territory, promote U.S. prosperity, preserve peace by renewing capabilities and working with allies, and increase U.S. influence by leading multilateral organizations.[7]In sum, both depend on maintaining or increasing U.S. relative advantage in several competitive domains – economic, political, military, multilateral, informational, space, cyberspace.
The Peloponnesian War and Relative Advantage
Geopolitical uncertainty and great power competition are nothing new. Neither is relying on relative advantage when facing them. Nearly 2,500 years ago, with the Peloponnesian War imminent, the Athenians—led by Pericles, the famous general-statesman—implemented such a strategy.
Athens was the dominant maritime power in Ancient Greece. Its unmatched navy extracted tributes,[8] grew the reserves, and ensured Athenian survival by securing port access to receive grain supplies from the Black Sea.[9] So with war looming, Pericles devised a strategy hinging on sea power that “would demonstrate to the Spartans and their allies that they were powerless to defeat Athens, and to exhaust them psychologically, not physically or materially.”[10] This strategy had two components.
First, the Athenians secured their long walls – and then retreated behind them. Seventeen miles around, twenty-five feet high, and ten feet thick, the long walls encircled Athens and Piraeus and protected a five hundred-foot wide road for the six miles from Athens to the sea.[11] This effectively made Athens an impenetrable island with secure port access. But with the Spartans free to pillage the now-abandoned Athenian territory outside the walls, Pericles defended his logic: “Dismissing all thought of our land and houses, we must vigilantly guard the sea and the city…We must cry not over the loss of houses and land but of men’s lives; since houses and land do not gain men, but men them.”[12]
Second, Athens would use the navy to launch raids “not meant to do serious harm but merely to annoy and harass the enemy and to give it a taste of how much damage the Athenians could do if they chose.”[13] Even as the Spartans pillaged Attica, Athens would not engage—the Spartans were stronger on land[14]—but demonstrate that it could raise the costs of war for Sparta from an untouchable position. In this way, Pericles hoped to swiftly conclude the war.
Both sides understood the deterrent effect of relative advantage. Archidamus, the Spartan king, warned that “unless we can either beat them at sea, or deprive them of the revenues which feed their navy, we shall meet with little but disaster.”[15] Pericles noted that “our naval skill is of more use to us for service on land, than their military skills for service at sea.” He also observed that for the Spartans, “familiarity with the sea they will not find an easy acquisition” and that “they will experience [hindrance] from want of money.”[16] Neither leader thus believed that Athens was likely to be defeated at sea – nor that Sparta had the time or resources to change that fact.
Evaluating the Athenian Strategy
How did Pericles’ strategy fare? The short answer: it failed. In the second year of the war, with over 300,000 Athenians behind the walls, Athens suffered a virulent plague[17] which killed nearly one-third of its population including armed forces.[18] The plague even took Pericles’ life in 429 after he was discredited, condemned, and blamed for bringing war to Athens.[19] Ultimately, after twenty-seven years of fighting, Athens lost the war, which also set the stage for further conflict in Ancient Greece.
The long answer is more complicated. Athens would have been defeated much earlier without the relative advantage conferred by its remarkable naval capabilities. Through Athens’ myriad setbacks—the plague, loss of Plataea, defeat at Amphipolis, failed peace of Nicias, defeat at Mantinea, Sicilian disaster, revolution, constitution changes, collapsed peace offerings—naval strength was the common factor prolonging its survival. In this regard, relative advantage worked.
In fact, by continuing to rebuild the fleet at great cost, even after devastating losses, Athens gained surprising opportunities for victory even when defeat seemed inevitable. At Cyzicus in 411, facing Sparta’s now-substantial naval fleet, on the brink of losing Black Sea access, and almost out of money,[20] Athens’ “excellent naval strategy” prevailed and decimated Spartan morale.[21] Five years later at Arginusae, the Spartans had rebuilt the fleet and acquired defected Athenian oarsmen while the weakened Athenians were destitute, melting golden statues around the Acropolis and emptying their reserves to fund their navy[22] - but Athens triumphed again with tactical ingenuity.[23] At Aegospotami in 405, Athens’ luck ran out and the Spartan navy finally prevailed. The long walls were dismantled, decisively ending the Athenian relative advantage.
Implications for U.S. Strategic Thinkers
How to reconcile this paradox, that the same strategy that failed initially also prolonged Athenian survival and even created surprising opportunities for victory? Moreover, what lessons can be drawn from the Peloponnesian War for today’s U.S. strategic thinkers, who ground the NSS and NDS in relative advantage?
(1) First, strategy is not perfect. States face seemingly limitless threats with finite resources and the budget constraints and trade-offs inherent in them. The strategy is then tested against the messiness and complexity of geopolitics and the chaos of war, where irritants and traps wait in the wings, prepared to upend even the safest strategic assumptions. It is impossible to fully anticipate or account for these factors when “the freaks of chance are not determinable by calculation.”[24] There are limits to what a strategy based on known quantities can guarantee for a future of unknown variables. So strategic thinkers must be humble and realistic about what strategy—at best—is equipped to achieve.
The Athenian experience provides a concrete example. Despite a clear relative advantage, Pericles could not have anticipated the plague or its impact,[25] which ultimately undermined Athenian confidence in his strategy and altered the course of the war. Nor could Athens anticipate Persia’s emergence to underwrite Sparta’s naval buildup. The longer a strategy is tested, it seems, the less safe become assumptions of relative advantage and the more damaging are the potential effects of chance and uncertainty.
This lesson is increasingly relevant for U.S. strategic planning today. The development of technology with apparently decisive strategic implications is accelerating. Cyber vulnerabilities, for example, are multiplying despite conscious efforts to counter them.[26] An adversary today can emulate and redeploy a newly-developed cyberweapon much more quickly than the Spartans produced their fleet. Other competitive domains echo this concern: the winner of the ‘new arms race’ toward hypersonic missiles could upend the logic behind traditional nuclear deterrence and quickly seize the advantage in the world’s military balance.[27] More than ever, relative advantage today is fleeting and eminently capable of evaporating.
Of course, imperfection is no grounds for abandonment. Just as relative advantage prolonged Athenian survival and even created surprising opportunities for victory, the U.S. must continue assessing vulnerabilities, bolstering capabilities, and building strengths. Although insufficient, relative advantage is necessary. But recalling these limits—finite means versus infinite ends, anticipating chance and uncertainty, maintaining relative advantage given the pace of technological development today—such a strategy is hardly a detailed roadmap guaranteeing security, but at best, a guide for navigating a geopolitical environment that will almost assuredly evolve in the context of great power competition.
(2) Second, implementation matters. There is a greater chance of mitigating the limits of strategy based on relative advantage with moderate and prudent implementation. While this may sound broad and somewhat nebulous, Thucydides is precise about what it entails in practice.
In Athens, moderate and prudent implementation meant prioritizing “the needs of the state” over “the whims of the multitude.”[28] This was possible with an exceptional leader like Pericles, a skilled orator who commanded widespread respect and led the masses of Athenian democracy instead of being led by them. So when he advised Athens to “wait quietly, to pay attention to their marine, to attempt no new conquests, and to expose the city to no new hazards during the war,”[29] they listened, even if chance derailed his strategy. Under his successors, the opposite happened: internal discord and private ambition poisoned strategic decision, producing blunders such as Sicily.
Moreover, relative advantage accelerated this downward spiral. Those who supported the “most costly and splendid Hellenic force” ever assembled sailing for Sicily,[30] or who rejected settlements for peace late in the war after Cyzicus and Arginusae,[31] were seduced by the perceived invincibility of the Athenian navy. Despite warning signs in each scenario—private squabbles in Athens which paralyzed support of the campaign in Sicily, and narrowly escaping defeat at Cyzicus and Arginusae—the Athenians continued the war rather than cutting their losses. Overestimating relative advantage can cause great powers to prolong engagement past the point of expediency, and consequently increase the long-term costs. The absence of moderation and prudence greases the path to defeat – particularly without an exceptional leader to control these tendencies.
This cautionary tale has implications for today’s U.S. strategic thinkers. Relative advantage can be wasted with hubris and presumed invincibility. The obvious solution is prioritizing state needs—particularly over incongruous popular demands and private interests—but this is no guarantee in practice, even under today’s U.S. system of checks and balances. Election calculations continue to wield significant influence over policy; what constitutes ‘state needs’ can be debated; and crisis situations leave little time for deliberation. More broadly, a relative advantage-based strategy also assumes that it will remain unchallenged and sufficient for achieving national security aims – which Sparta undermined by persistently invading Attica throughout the war and eventually acquiring a navy. Thus, moderate and prudent implementation also entails being prepared to reassess strategic assumptions as they are challenged and considering one’s own weaknesses—not only strengths—relative to the competition. However, this is difficult in practical terms, as reassessing strategic assumptions serves little purpose if deterrence has already failed.
And Thucydides offers not elegant solutions, only the convergent counsel of two competing statesmen. As Pericles may have advised today’s strategic thinkers, “wait quietly, mind your own strengths, do not overstretch, and avoid hazards.”[32] Similarly, Archidamus would caution: “proceed with wise moderation, perfect your own preparations, assume your adversary’s plans are not dissimilar or inferior to yours, and rest your hopes not in enemy blunders, but the soundness of your own provisions.”[33] Strategy is complicated but these insights into moderate and prudent implementation can temper its limitations and help avoid its pitfalls.
(3) Third, U.S. strategic thinkers should build the strongest possible bulwarks against the limits of strategy. This means ‘setting the theatre’—the pre-crisis environment in which strategic decision occurs—to encourage more moderate and prudent implementation.
Here, it is useful to draw distinctions between Ancient Greece and the post-WWII international order. In the Peloponnesian War, alliances shifted frequently and altered the course of the war, notably with Persia’s arrival. By contrast, the U.S. today benefits from international institutions and long-standing alliances which promote stability and strengthen ties among like-minded states. Notably, for over seventy years, NATO has offered the U.S. unprecedented alliance stability and therefore greater deterrent effect against geopolitical competitors. Robust alliances also encourage moderate and prudent strategic behavior because they are more insulated from domestic politics, offer support against unanticipated events,[34] and provide a mechanism for crisis decision-making. And while members can opt-in or out of alliances at will, increasing the U.S. commitment to existing alliances and promoting greater cohesion within them will only benefit U.S. strategic interests by incentivizing other members to follow and thereby increase the deterrent effect perceived by geopolitical competitors. As Archidamus testified, great powers are more secure with allies: perfecting Sparta’s preparations before the Peloponnesian War meant “first, the acquisition of allies…so long as they are an accession to our strength naval or financial.”[35] The U.S. should build on the advantage that exists already.
Perhaps the greatest lesson imparted by Thucydides is about remembering what war entails. As Archidamus observed in 432, those who have “had the experience of many wars…will not fall into the common misfortune of longing for war from inexperience or from a belief in its advantage and its safety.”[36] Thucydides’ subsequent narrative is a vigorous defense of this insight, from the horrors of Corcyra; to Athens’ fall from exceptional civilization to executing the neutral Melians; to the final naval battles, where both sides having opportunities to win suggests that the previous decades of devastation tragically produced little more than stalemate. The potential costs of great power conflict are so pernicious, even for the mighty, that engagement must be chosen carefully and only after exhausting all other options. And while seemingly obvious—but unusually compelling as a dispassionate but solemn observation of reality—it is the foundational insight on which the art of strategic decision should rest. When navigating today’s fragile geopolitical environment, there is a far greater chance of strategic success if today’s generation recalls this wisdom.