What are the reasons for Japan’s easy victories in the initial period of the war?

It’s about World War II. At the initial stage (from 1937 or 1939, depending on how you count), the Japanese fought with the Chinese, and they had a weak army and internal strife between the communists and republicans.

Since late 1941, the Japanese have fought against weak opponents in Southeast Asia. For example, in the Dutch colony of Indonesia, the number of all garrisons was about 10 thousand people, and the local population had no sympathy for the colonialists. Similarly, in the former French Indochina and in the Philippines and in British Malaysia, where almost all of them immediately surrendered.

The Thai leadership took a pro-Japanese position, and the British were quickly driven out of Burma.