Soon after in April Cassel was employed alongside James R. Wasson to serve on a punitive expedition against the aborigines of Taiwan. Cassel and Wasson set off on the ship "Nepaul" with the expedition's advance party of 100 men, under Cassel's command. They landed on Taiwan May 6, 1874, selecting a beachhead campsite. On May 15, Cassel lead negotiations with Chief Issa, head of the island's 16 tribes, where Issa gave the Japanese permission to punish the offending Botan tribe. A celebratory feast was had, where a Winchester rifle, brought along by either Cassel or Wasson, was used to impress the friendly natives with its capacity.