Loot a burning house
chèn huǒ dǎ jié
Use the opportunity of fire to rob others.
When a country is beset by internal conflicts and/or disease and famine devastates the population and/or when corruption and crime are rampant - it will be unable to deal with an outside threat. This is the time to attack.
Keep gathering internal information about your enemy. Once he is in his weakest state ever, attack without mercy and totally destroy him to prevent future problems.
Cause the enemy problems which he is required to deal with. Then attack him when he is distracted.
When Qin and Wei attacked Han, Han asked Qi for help. Qi agreed but actually didn't give the promised help, as other allies Chu and Chao stepped in. This left Yan, another state which was in internal turmoil, without support. Qi seized the opportunity to attack and conquered Yan, whose allies were now busy elsewhere.
After a humiliating defeat in 493 BC by Fu Chai of Wu State, Gou Jian of Yue State gave a large amount of luxuries to Fu Chai to encourage his licentious ways. He also provided sterilized rice seeds to ensure a famine outbreak. When Fu Chai went on a journey, Gou Jian easily conquered the weakened and unhappy state. On his return, Fu Chai begged for mercy. He was only offered execution or suicide, of which he chose the latter.