Balance of Power
The oldest working idea in European statecraft: when one state gets strong enough to dominate, the others gang up to stop it.
The balance of power is the idea that in a world of independent states, any country strong enough to dominate the rest will provoke a coalition against it, and that this reflex is what keeps everyone else independent. It is both a description of what tends to happen and a policy governments have followed deliberately for five hundred years.
The practice is older than the phrase. Renaissance Italy's five competing states produced the shifting alliances and permanent ambassadors that Europe later copied on a continental scale. The term entered treaty law at Utrecht in 1713, which justified the settlement ending the War of the Spanish Succession as establishing a just equilibrium of power. Europe had just spent thirteen years making sure France and Spain would not be ruled by the same king.
How states do it
A country can balance internally, by building up its own army and industry, or externally, by finding allies. The alternative is to join the strong side instead of resisting it, which small states with no chance of survival often do. A third option, and a popular one, is to agree that somebody ought to stop the rising power and hope it turns out to be somebody else's army. Britain and France did that to each other through most of the 1930s.
Britain turned balancing into a national doctrine. It had no ambition to rule the continent and a navy that kept it separate from it, so it repeatedly joined whichever coalition opposed the strongest continental power: against Spain, then Louis XIV, then Napoleon, then Germany twice. Churchill described the policy as always joining the less strong side and combining against the military tyrant of the moment, whoever he happened to be.
The system worked best after 1815. The powers that beat Napoleon designed the Vienna settlement to make another bid for domination difficult, then met periodically to manage crises. It was hostile to revolution and it did not prevent every war, but no conflict involving all the great powers happened in Europe again until 1914.
What later thinkers added
Kenneth Waltz argued that balancing is not really a policy at all. In a world with no authority above states, balances form whether anyone intends them or not, the way competing firms in a market end up behaving alike.
Stephen Walt then added the correction that does most of the explaining: states balance against threat, not against raw strength. Distance, offensive capability, and what a country appears to intend all matter. That is why the world did not gang up on the United States after 1991, and why it is a genuine question whether Asia is balancing China now or accommodating it.
Balancing also fails in two ways. It can produce the war it was supposed to prevent, which is what the rival alliance blocs did in 1914, turning a Balkan quarrel into a continental catastrophe. Or it can simply not happen, as in the 1930s, when every country threatened by Hitler waited for someone else to move first.
Nobody in Washington, Beijing, Delhi, or Brussels uses the phrase in a speech any more. But AUKUS, the Quad, Japanese rearmament, Finland and Sweden joining NATO, and Chinese diplomacy aimed at pulling those groupings apart are the same behavior European monarchies were practicing three centuries ago.