In the first century BC, the Roman senator and historian Sallust looked back at the collapse of the Roman Republic and tried to identify the moment things started going wrong. He settled on 146 BC, the year Rome destroyed Carthage after the Third Punic War. For more than a century, the Roman ruling class had been held together by the shared fear of the Carthaginian rival across the Mediterranean. Once Carthage was flattened, that discipline evaporated. Within a generation, Roman politics slid into the civil wars — the Gracchi, Marius against Sulla, eventually Caesar against Pompey — that ended the Republic. Sallust's diagnosis, distilled from decades of watching Roman senators tear each other apart, was blunt: metus hostilis, 'fear of the enemy', was what had kept the Republic honest. Take the enemy away, and the alliance eats itself. That one observation has echoed through political thought for two thousand years.
Modern international-relations scholars — from Hans Morgenthau to Stephen Walt — have generalised Sallust's insight into what is sometimes called, half-jokingly, Sallust's theorem: alliances hold together as long as the threat posed by their shared enemy is larger than the threats members pose to each other. When the shared threat fades, internal rivalries return. This is not a law of nature. It is more like gravity — constantly there, easily forgotten during good weather, and immediately obvious the moment things start falling. Walt's 'balance of threat' theory (1987) formalised it: states ally not against power in the abstract but against the specific power they most fear. Which means the durability of any alliance is not really about shared values, treaties, or speeches. It is about who both sides are afraid of, and how afraid they are, and whether that answer is roughly the same.
The North Atlantic Treaty Organisation was founded in April 1949 with twelve members and one very clear enemy: the Soviet Union, whose forces had just blockaded Berlin, absorbed Eastern Europe, and detonated their first atomic bomb four months later. For the next forty years, that enemy did all the work of holding the alliance together. France quarrelled bitterly with the US, briefly withdrew from NATO's integrated command in 1966, and pursued its own nuclear force. Germany was rearmed against the deep instincts of every neighbour who had been invaded by it. Turkey and Greece almost went to war three times, most seriously over Cyprus in 1974. None of it broke the alliance. Every serious internal fight was ultimately settled by the same argument: whatever we think of each other, we cannot afford to break up while the Red Army is sitting in East Germany. Moscow was the glue. The founding NATO Secretary-General Lord Ismay's famous summary — 'keep the Russians out, the Americans in, and the Germans down' — was really a description of what the shared threat allowed the alliance to do.
When the Soviet Union collapsed in December 1991, so did NATO's obvious purpose. The alliance spent the 1990s and 2000s in a long, sometimes bitter argument about what it was actually for. Was it a democracy-promotion vehicle? A humanitarian-intervention force (Bosnia 1995, Kosovo 1999)? A counter-terror coalition (Afghanistan after 2001)? An out-of-area expeditionary tool (Libya 2011)? Membership grew rapidly — twelve new states between 1999 and 2020 — but cohesion drifted. Public support for defence spending fell across almost every European member. American administrations from Bush I onward complained openly that Europe was not paying its share. Every Trump-era demand about the 2% GDP defence-spending target lives directly in this gap: without a clear enemy, the burden of the alliance felt discretionary, and discretionary spending is always the first to be cut. Sallust could have predicted the fight almost word for word.
Russia's full-scale invasion of Ukraine on 24 February 2022 did what a decade of American lecturing about burden-sharing could not. Within three days, Germany's chancellor Olaf Scholz stood in the Bundestag and announced a special €100 billion defence fund, promised to hit the 2% target permanently, and used the word 'Zeitenwende' — turning point. Within four months Finland and Sweden had formally abandoned two centuries of neutrality (Finland's since 1948, Sweden's since 1814) and applied to join NATO. Poland's defence spending rocketed past 4% of GDP and is now building the largest army in NATO Europe, with over 1,000 K2 tanks on order from South Korea. The Baltic states doubled their defence budgets. Even Switzerland — famously, foundationally neutral — began quietly co-operating with the European Sky Shield air-defence initiative. The theorem worked on schedule. Take the shared threat away, and NATO drifts; put it back, and the alliance re-cohered within weeks.
The rule cuts in both directions, and this is where it gets uncomfortable. Russia, China, Iran and North Korea have almost nothing in common ideologically — an ex-communist kleptocracy, a Marxist-Leninist party state, a Shia theocracy, and a Stalinist dynastic monarchy. They agree on very little. But they are increasingly pulled together by a single shared antagonist: the American-led international order and the sanctions, freedoms and alliances that come with it. Since 2022, the practical co-operation among the four has accelerated in ways that would have been dismissed as fantasy in 2015. China provides Russia with dual-use goods and diplomatic cover; Russia sells oil to China at a discount and to India at a slightly smaller one. Iran has shipped thousands of Shahed drones to Russia and gets sanctions-evasion help in return. North Korea has sent millions of artillery shells, ballistic missiles and, since October 2024, actual soldiers to fight in Kursk. This is not a formal alliance — no treaty, no headquarters — but it functions like one, and its concord has exactly the same source as NATO's: a shared enemy larger than their differences.
If Sallust is right — and two millennia of evidence suggest he mostly is — the durability of the transatlantic alliance over the coming decade depends less on speeches, summits, or treaty language than on the map. As long as Russia menaces Europe and China contests the Pacific, the pull toward cohesion is real and will keep supplying the political oxygen for higher defence spending, expanded manufacturing, and closer intelligence-sharing. But the theorem has an unforgiving corollary. The moment either threat is perceived to have gone away — whether because it actually recedes, or because it is dismissed as manageable, or because a rival power successfully persuades one side that peace is close — the theorem starts working in reverse. Internal fights that were suppressed for a generation come flooding back. The 1990s were not an aberration; they were the theorem in action. Watch the internal fights. The next round is being previewed right now, in every argument between Washington and Brussels about Ukraine, tariffs, and Article 5.
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