1941, Koo was appointed Chinese ambassador in London. Many in the Foreign Office disliked Koo as one of Britain's "bitterest enemies" in the 1920s, charging that he was a Chinese nationalist who rejected the "white man's burden" view of Britain's role in the world. Koo's main role in London was in seeking British support for China and especially to ensure that the British did not close the Burma Road, which was China's lifeline following the closure of the French railroad to China. In July 1941, the Japanese occupied the southern half of French Indochina, giving Japan the ability to project air and naval power well into the South China Sea. In response, the American, British and Dutch governments imposed an oil embargo on Japan and froze all Japanese assets in their countries. The Foreign Secretary Anthony Eden chose not to inform Koo of these steps, who first learned of these British moves from reading the newspapers. Koo was deeply worried about a possible Japanese invasion of Burma, which would close the Burma Road. Koo pressed for more British aid to China, only to be told that the British were fully committed to the struggle against Germany and had little to spare to assist China. In the summer and fall of 1941, Koo offered to have Chinese troops sent to reinforce the garrison in Hong Kong, only to be politely rebuffed as the Foreign Office told him that the existing Anglo-Indian-Canadian garrison could hold Hong Kong against any Japanese attack. Following the Japanese attack on the British empire in December 1941, the Japanese invaded the British colony of Burma in January 1942 from Thailand. As an ambassador in London, Koo played a critical role in the talks that saw the British grant the Chinese permission to send two armies into Burma to assist the British in the defense of Burma. The British prime minister, Winston Churchill, was opposed to accepting the Chinese offer, preferring that Burma be defended by only British troops or Indian troops under the command of British officers. Much to Koo's pleasure, the dire situation in Burma with the British being defeated by an invading Japanese force that consisted of only 2 divisions forced Churchill to accept the Chinese offer of help. For Koo, the fact that Britain was being forced to accept Chinese help symbolized how the Second World War was changing the face of world politics.