Please answer a question about the following article about Pacific War:

The Axis states which assisted Japan included the authoritarian government of Thailand in World War II, which quickly formed a temporary alliance with the Japanese in 1941, as the Japanese forces were already invading the peninsula of southern Thailand. The Phayap Army sent troops to invade and occupy northeastern Burma, which was former Thai territory that had been annexed by Britain much earlier. Also involved were the Japanese puppet states of Manchukuo and Mengjiang (consisting of most of Manchuria and parts of Inner Mongolia respectively), and the collaborationist Wang Jingwei regime (which controlled the coastal regions of China).

What army invaded northeastern Burma?
Phayap Army