When Enemies United
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When Enemies United
Greater Threats That Forced History's Fiercest Rivals To Align
Union Against The Great King
Rival Grek-City States Facing The Persian Empire As One
Ancient Greece was not a unified nation but a collection of independent city-states, each with its own government, ambitions and interests. Throughout the late sixth and early fifth centuries BCE, particularly following the reforms of Cleisthenes in 508/507 BCE, Athens emerged as a growing democratic power, while Sparta remained a militarised oligarchy under its dual kings. Their contrasting political systems, military priorities and competing influence fostered an enduring rivalry. While they occasionally cooperated, distrust and competition were far more common, leaving Greece politically fragmented on the eve of one of the greatest invasions in its history.
That rivalry was overshadowed by the growing threat of the Achaemenid Persian Empire under King Xerxes I, who launched his invasion of Greece in 480 BCE. Stretching from the Indus Valley to Egypt, Persia dwarfed the Greek city-states in territory, population and resources. Recognising that no single polis could hope to withstand such a powerful empire alone, Themistocles helped persuade a number of Greek states to form the Hellenic League in 481 BCE. Their temporary partnership was driven not by friendship but by the shared understanding that continued division would likely end in conquest.
The alliance fought together during the Second Persian Invasion of Greece, achieving decisive victories at Thermopylae, Salamis and Plataea under leaders including King Leonidas I, Themistocles and Pausanias, ultimately securing Greek independence by 479 BCE. Once the immediate Persian threat had passed, however, the unity quickly unravelled. Old rivalries resurfaced, culminating in the Peloponnesian War (431–404 BCE) as Athens and Sparta once again turned their attention towards each other. Yet Greece would not be the last place where long-standing enemies discovered that survival sometimes demanded cooperation before conflict inevitably returned.
An Unlikely Coalition
Roman-Germanic Conflict Halting In The Face Of The Huns
Centuries later, another unlikely partnership emerged on the fading frontiers of the Western Roman Empire. Since the first century BCE, Rome had fought numerous Germanic peoples in conflicts that ranged from frontier raids to full-scale invasions. The disastrous Battle of the Teutoburg Forest (9 CE), where the chieftain Arminius destroyed three Roman legions, demonstrated the scale of this hostility. More recently, the Battle of Adrianople (378 CE) saw the Visigoths inflict one of Rome's greatest defeats, while the Sack of Rome (410 CE) under Alaric I deepened the long-standing mistrust between the empire and the Germanic kingdoms.
By 450 CE, however, a far greater threat had emerged in the form of Attila the Hun. Having forged a vast confederation stretching across much of Central and Eastern Europe, Attila launched increasingly aggressive campaigns against both the Eastern and Western Roman Empires. As his armies advanced into Gaul in 451 CE, it became clear to the Roman general Flavius Aetius that neither Rome nor the neighbouring Germanic kingdoms could realistically resist the Huns alone. Faced with a common danger, Aetius assembled a coalition that included the Visigoths under King Theodoric I, setting aside decades of conflict in favour of mutual survival.
The coalition confronted Attila at the Battle of the Catalaunian Plains (451 CE), where the Hunnic advance into Western Europe was halted despite the death of Theodoric I during the fighting. The alliance proved that even centuries of hostility could be suspended when both sides faced a sufficiently powerful enemy. Yet the cooperation was only ever temporary. Following Attila's death in 453 CE and the collapse of Hunnic power, the old tensions between Romans and Germanic kingdoms quickly resurfaced, contributing to the continued fragmentation of the Western Roman Empire before its eventual fall in 476 CE. As Europe entered a new age, the same pattern of necessity would emerge once again under a different threat.
Joining To Lock The Gates of Europe
Religious and Political Oppnents Standing Together Against The Ottomans
As Europe entered a new age, rivalries between its Christian powers remained deeply entrenched. Throughout the sixteenth and seventeenth centuries, disputes over territory, dynastic succession and religion repeatedly brought European states into conflict. The Thirty Years' War (1618–1648) had devastated much of Central Europe, pitting Catholic and Protestant states against one another, while powers such as the Habsburg Monarchy and the Polish–Lithuanian Commonwealth often pursued competing political interests. By the late seventeenth century, Europe remained divided despite sharing a common civilisation.
That changed when the Ottoman Empire, under Grand Vizier Kara Mustafa Pasha, launched a major campaign into Central Europe. In 1683, the Ottomans laid siege to Vienna, threatening the Habsburg capital and potentially opening the way for further expansion into the continent. Recognising the scale of the danger, Holy Roman Emperor Leopold I and King John III Sobieski of Poland forged a broader defensive partnership with other European powers. Their cooperation was driven less by mutual trust than by the realisation that the Ottoman advance posed a threat to them all.
The allied forces broke the Siege of Vienna (1683) before continuing their cooperation throughout the Great Turkish War (1683–1699), achieving further victories at battles such as Mohács (1687) and Zenta (1697). The conflict concluded with the Treaty of Karlowitz (1699), which significantly reduced Ottoman influence in Central Europe. Yet once the immediate danger had passed, Europe's internal rivalries soon returned, with dynastic and territorial disputes once again dominating relations between the continent's powers. More than a century later, another dominant force would compel Europe's rivals to cooperate once again.
Combining To Defeat Napoleon
Longtime European Fighters in Unison to Overthrow France's Emperor
More than a century later, Europe's rival powers again found themselves divided by long-standing competition. Throughout the eighteenth century, states including Austria, Prussia, Russia and Great Britain had repeatedly fought one another in conflicts such as the War of the Austrian Succession (1740–1748) and the Seven Years' War (1756–1763). Dynastic ambition, territorial expansion and the balance of power ensured that cooperation was usually short-lived, with alliances shifting as quickly as political interests changed.
That changed as Napoleon Bonaparte transformed France into the dominant power on the European continent. Following victories at battles such as Austerlitz (1805) and Wagram (1809), much of Europe either fell under French control or was forced into alliance with France. Napoleon's disastrous invasion of Russia in 1812, however, created an opportunity for his opponents. Recognising that none could permanently curb French dominance alone, Tsar Alexander I, King Frederick William III of Prussia, Emperor Francis I of Austria and the British government formed the Sixth Coalition in 1813.
The coalition secured a decisive victory at the Battle of Leipzig (1813) before continuing the campaign into France, forcing Napoleon's abdication in 1814. After his brief return during the Hundred Days (1815), the allies defeated him once and for all at the Battle of Waterloo, led by Arthur Wellesley, Duke of Wellington, and Gebhard Leberecht von Blücher. Although Europe's rivals had combined their strength to defeat a common enemy, their partnership quickly gave way to renewed competition over influence and territory. Little more than a century later, the same logic of survival would compel bitter rivals on the other side of the world to suspend an entirely different conflict.
Civil War Paused
Bitter Chinese Competitors Switching Attention to Resist Imperial Japan
Little more than a century later, another alliance of necessity emerged in East Asia. Following the collapse of the Qing dynasty in 1912, China became increasingly fragmented as competing political movements struggled for control of the country. By 1927, the Kuomintang under Chiang Kai-shek and the Chinese Communist Party led by Mao Zedong had descended into open civil war. Campaigns such as the Communist Long March (1934–1935) deepened the hostility between the two sides, making reconciliation appear impossible.
That conflict was eclipsed by the growing threat of Imperial Japan. Following the Mukden Incident (1931), Japan occupied Manchuria before launching a full-scale invasion of China in 1937 after the Marco Polo Bridge Incident. The turning point came with the Xi'an Incident (1936), when Zhang Xueliang detained Chiang Kai-shek and pressured him into cooperating with the Communists against the external threat. The resulting Second United Front reflected necessity rather than trust, as both sides recognised that a divided China stood little chance against Japan's modern military.
Although tensions never disappeared, Nationalist and Communist forces fought on the same side throughout the Second Sino-Japanese War (1937–1945), enduring battles such as Shanghai (1937) and Wuhan (1938) while resisting Japanese occupation. Japan's defeat in 1945 removed the common enemy that had held the alliance together, and the Chinese Civil War quickly resumed before ending with the Communist victory in 1949. Like every case before it, this partnership proved that even the fiercest rivalries could be suspended when survival demanded it—but only for as long as the greater threat remained.
Necessity Over Grudge
Across more than two thousand years of history, these alliances followed a remarkably consistent pattern. Bitter rivals that had spent years, decades or even centuries competing for power temporarily abandoned their differences when confronted by a greater external threat. Whether in ancient Greece, the late Roman world, early modern Europe, Napoleonic Europe or twentieth-century China, cooperation emerged not from trust or reconciliation but from necessity. Yet just as consistently, those partnerships dissolved once the common enemy had been defeated, with old rivalries quickly resurfacing. History suggests that while a shared danger can unite even the fiercest enemies, it rarely changes the relationship between them once that danger has passed.
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