Why was a small country like Japan able to defeat and completely overrun a behemoth nation like China in World War II?

 The question contains inaccuracies. Japan did not completely overrun China during World War II. In fact, China was one of the major theaters of conflict during the war, and Japan faced significant challenges in its attempt to conquer China.

The Second Sino-Japanese War, which began in 1937, predates World War II, and it was a protracted conflict between Imperial Japan and the Republic of China. Several factors contributed to the challenges and complexities of the war:

1. **Size and Population:**

   - China is a vast country with a large population, making it challenging for any invader to completely overrun. The sheer size of China presented logistical difficulties for the Japanese military.

2. **Guerilla Warfare:**

   - The Chinese military, led by the Nationalist government under Chiang Kai-shek, adopted guerilla warfare tactics, which proved effective against the conventional strategies employed by the Japanese. This allowed Chinese forces to continue resistance even after losing major cities.

3. **International Support:**

   - Despite Japan's initial military successes, China received international support, particularly from the Soviet Union and, later in the war, from Western Allies. This support included military aid and supplies, which helped sustain Chinese resistance.

4. **Terrain and Infrastructure:**

   - The diverse and challenging terrain of China, from dense jungles to vast deserts, made military operations difficult for the Japanese. Additionally, China's vastness meant that capturing and maintaining control over key strategic locations was a complex task.

5. **Resource Strain:**

   - Japan's expansionist policies and involvement in multiple theaters, including China and later the Pacific, stretched its military resources thin. The need to allocate resources to various fronts impacted Japan's ability to sustain a prolonged campaign in China.

While Japan occupied certain regions of China during the war, it did not achieve complete control over the entire country. China's resilience, international support, and the logistical challenges faced by the Japanese military contributed to the ongoing resistance in China throughout World War II. The conflict in China continued until Japan's surrender in 1945, and the conclusion of the war saw the restoration of Chinese sovereignty.