Definition
An alliance system is a network of formal agreements in which states pledge to support one another, usually militarily, if any member is attacked. The term most often refers to the rival blocs of early twentieth-century Europe, where the Triple Alliance and the Triple Entente divided the great powers into two armed camps.
These commitments were meant to deter aggression by guaranteeing that an attack on one nation would draw in others. Instead, the same interlocking promises helped transform a single assassination in 1914 into a continent-wide war.
Why it matters
How it works
In an alliance system, each state calculates that the cost of standing alone is greater than the cost of binding itself to partners. The promises are reciprocal, so a chain of obligations forms: if A is attacked it calls on B, whose entry obliges C, and so on. When a crisis erupts, military planning that depends on speed — mobilizing troops before the enemy does — pressures every government to act quickly rather than wait. The deterrent that was supposed to keep peace becomes a tripwire that spreads war.