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The Thucydides trap is the idea that war becomes highly likely when a rising global power threatens to displace an established ruling power. It is named after the ancient Greek historian Thucydides, who wrote about the Peloponnesian War between Athens and Sparta. His argument was simple. The rise of Athens created fear in Sparta, and that fear helped make war almost inevitable.

That is why the phrase now matters so much in the rivalry between China and the US.

China is the rising power. The US is the established power. Beijing wants a greater share of global influence, economic weight, military reach, and diplomatic respect. Washington wants to preserve the system it built and dominated after the Second World War. The danger is that each side reads the other’s actions with suspicion.

A Chinese move in the South China Sea may be seen in Washington as expansionism. An American military deployment near Taiwan may be seen in Beijing as encirclement. A trade dispute may no longer look like a dispute over tariffs, but as part of a much larger struggle over global dominance.

That is the trap.

According to historical research by Harvard’s Belfer Center, this kind of structural power shift has occurred 16 times over the past 500 years. In 12 of those cases, the result was devastating war. The lesson is not that war is guaranteed. The lesson is that the margin for error becomes dangerously small.

Once an established power becomes paranoid about decline, and a rising power becomes impatient for recognition, diplomacy starts to operate under intense pressure. Even a relatively minor incident can become a trigger. A naval collision, a sudden crisis over Taiwan, a trade confrontation, or a technological dispute can escalate far beyond its original cause if both sides already believe they are locked in a struggle for supremacy.

Xi’s warning to Trump therefore carries a strategic message. China is effectively saying that America must not treat China’s rise as something that can simply be blocked, contained, or reversed. The US, meanwhile, will not accept a world in which China uses its rise to intimidate neighbours, rewrite trade rules, or challenge American power without consequence.

That is what makes this moment so dangerous.

The Thucydides trap does not say that war must happen. Four of the 16 historical cases studied avoided war through major diplomatic compromise. But that required both sides to understand the fear driving the other, and to manage flashpoints before they spiralled into catastrophe.

The Common Sense continues to argue the point that the real battlefield between China and America of the next twenty years will be a duel over the reserve status of the dollar.

Examples of the Thucydides trap from the Belfer Center:

1. Late 15th century — Portugal vs Spain — NO WAR

2. First half of 16th century — France vs Hapsburgs — WAR

3. 16th and 17th centuries — Hapsburgs vs Ottoman Empire — WAR

4. First half of 17th century — Hapsburgs vs Sweden — WAR

5. Mid-to-late 17th century — Dutch Republic vs England — WAR

6. Late 17th to mid-18th centuries — France vs Great Britain — WAR

7. Late 18th and early 19th centuries — United Kingdom vs France — WAR

8. Mid-19th century — France and United Kingdom vs Russia — WAR

9. Mid-19th century — France vs Germany — WAR

10. Late 19th and early 20th centuries — China and Russia vs Japan — WAR

11. Early 20th century — United Kingdom vs United States — NO WAR

12. Early 20th century — United Kingdom (supported by France, Russia) vs Germany — WAR

13. Mid-20th century — Soviet Union, France, and United Kingdom vs Germany — WAR

14. Mid-20th century — United States vs Japan — WAR

15. 1940s-1980s — United States vs Soviet Union — NO WAR

16. 1990s-present — United Kingdom and France vs Germany — NO WAR