Reacting to Japanese pressure on French authorities of French Indochina to stop trade with China, the US began restricting trade with Japan in July 1940. The end of all oil shipments in 1941 was decisive since the Americans, British and Dutch provided almost all of Japan's oil. In September 1940, the Japanese invaded Vichy French Indochina and occupied Tonkin to prevent China from importing arms and fuel through French Indochina along the Sino-Vietnamese Railway from the port of Haiphong through Hanoi to Kunming, in Yunnan. The US decided that the Japanese had now gone too far and decided to force a rollback of its gains. In 1940 and 1941, the Americans and the Chinese decided to organise a volunteer squadron of American planes and pilots to attack the Japanese from Chinese bases. Known as the Flying Tigers, the unit was commanded by Claire Lee Chennault. Its first combat came two weeks after the attack on Pearl Harbor.