how did the war of the five kings start

The War of the Five Kings starts with a chain reaction of political betrayal after King Robert Baratheon dies, but the immediate spark is Catelyn Stark seizing Tyrion Lannister, which turns a tense feud into open conflict.

What set it off

• Robert Baratheon’s death leaves the realm unstable, with competing claims and rival power blocs already in place.

• Catelyn Stark arrests Tyrion Lannister, accusing the Lannisters of being behind the attack on Bran Stark.

• Tywin Lannister responds by sending raids into the riverlands, which escalates the dispute into war.

• Soon after, Jaime Lannister leads troops into the riverlands and fighting begins in earnest.

Why it spread so fast

The conflict grows because multiple rulers see a chance to press their claims while the central authority collapses. That is why it becomes a multi-sided war rather than a simple Stark-versus-Lannister feud.

In one line

It began as a political arrest and family vendetta, then exploded into a wider civil war once the Lannisters retaliated and other claimants moved in.

TL;DR

Catelyn Stark’s capture of Tyrion is the immediate trigger, but Robert’s death and the scramble for power are what turn that trigger into the War of the Five Kings.