In October 1592, Katō Kiyomasa decided to attack a nearby Jurchen castle across the Tumen River in Manchuria to test his troops against the "barbarians", as the Koreans called the Jurchens. Kato's army of 8,000 was joined by 3,000 Koreans, at Hamgyong, because the Jurchens periodically raided across the border. Soon the combined force sacked the castle, and camped near the border; after the Koreans left for home, the Japanese troops suffered a retaliatory assault from the Jurchens. Katō Kiyomasa retreated with his forces to avoid heavy losses. Because of this invasion, rising Jurchen leader Nurhaci offered military assistance to the Joseon and Ming in the war. However, the offer was refused by both countries, particularly Joseon, saying that it would be disgraceful to accept assistance from the "Barbarians" to the north.