In September 1940, Japan decided to cut China's only land line to the outside world by seizing Indochina, which was controlled at the time by Vichy France. Japanese forces broke their agreement with the Vichy administration and fighting broke out, ending in a Japanese victory. On 27 September Japan signed a military alliance with Germany and Italy, becoming one of the three Axis Powers. In practice, there was little coordination between Japan and Germany until 1944, by which time the U.S. was deciphering their secret diplomatic correspondence.
When did Japan invade Indochina?
September 1940