Loot a Burning House
When a country is beset by internal conflicts, when disease and famine ravage the population, when corruption and crime are rampant, then it will be unable to deal with an outside threat. This is the time to attack.
Warring States Era China
Qi and Han were allies when Chang Yi attacked Han with the combined forces of Qin and Wei. Han asked Qi for assistance. The king of Qi said:" Han is our ally and since Qin has attacked her we must go to her rescue." But his minister Tian-chen Su disagreed saying: "Your majesty's planning is faulty. You should merely agree to assist Han but take no action there. However, in the kingdom of Yan, their king has recently resigned the throne to his despised prime minister. This has enraged both the noble houses and the common people causing turmoil at court. Now if Qin attacks Han, Chu and Chao will surely come to her aid and this will be as good as heaven bestowing Yan upon us."
The king approved and promised the Han envoy assistance before sending him back to Han believing he had Qi's backing. When Qin attacked Han, Chu and Chao intervened as expected. While all the major kingdoms were thus engaged in the battle for Han, Qi quickly and quietly attacked Yan. Within thirty days Yan was captured.