When Toyotomi Hideyoshi invaded Korea in 1592, Ming China quickly sent a huge army to Korea to help its vassal. The consequent seven-year war fought by China and Korea against Japan was the largest war in the entire 16th-century world.
One little known fact is that Siam or Thailand, which was also a Chinese vassal, offered to send an enormous fleet (supposedly involving 200,000 troops) to attack the Japanese Archipelago in order to draw Japanese troops away from Korea.
This anger happened, but what if it had?