Why did the Yongle emperor threaten a punitive expedition against Japan?

The Yongle emperor threatened a punitive expedition against Japan primarily as a response to the resurgence of piracy along the Chinese coast.1

While the Japanese shogunate had initially maintained a tributary relationship with the Ming dynasty—even going so far as to send subjects to the Chinese court for punishment as pirates—this cooperation ceased following a change in leadership. From 1411 onward, Japan stopped sending tribute missions and Japanese raiders became increasingly active in plundering the Chinese coast.1

The emperor issued the threat to compel Japan to reform its behavior and resume its former tributary status. However, when the Japanese shogunate brusquely denied responsibility for the piracy and refused to return to the tributary relationship in 1419, the Yongle emperor was preoccupied with other matters and did not carry out the expedition.1