This idiom comes from a story during the Warring States period.
Zhao Kuo was the son of a famous general.
He studied war books, knew every strategy by heart, and loved to talk about military theory.
On paper, he was brilliant.
One day, his country put him in charge of a real army.
But on the battlefield, he made big mistakes.
He followed theory — not real-life judgment.
The enemy surrounded his army.
Over 400,000 soldiers were lost.
Zhao Kuo died in battle.
That became a metaphor:
Talking about strategy is easy — but without experience, it can lead to disaster.