What event cause Japan to invade Manchuria?

The event that caused Japan to invade Manchuria was the Mukden Incident.

Explanation:

The Mukden Incident occurred on September 18, 1931, near the city of Mukden (today Shenyang) in Chinese Manchuria and became the beginning of the Japanese invasion and occupation of the region. The incident was a bomb that exploded at the Japan-controlled railway. Japan accused Chinese extremists of the attack and sent soldiers to the area. A few months later, Japan had conquered all of Manchuria, which was then made into a Japanese sound state - Manchukuo.

Japan invaded Manchuria for the need of natural resources.

Furthermore, it was occupied by Russia, and Japan felt threathened.