Zhang organized a military government. After the Northern Expedition toppled the government in Beijing in 1928, he was briefly wanted for arrest by the new Nationalist government in Nanking, but through Chang Hsueh-liang's mediation he was reconciled with the new government and returned to the diplomatic service. He represented China at the League of Nations to protest the Japanese invasion of Manchuria. Much to Koo's fury, the British Foreign Secretary Sir John Simon, was able to insist the League not take action against Japan, and instead appoint a commission to examine whatever Japan had committed aggression or not. Koo was assigned to the League's Lytton Commission-so called after its chairman, Lord Lytton-that was sent in December 1931 to go to Manchuria to investigate if Japan was an aggressor or not. Ambassador in Paris In August 1932, Koo arrived in Paris as the Chinese minister-plenipotentiary. Koo also concurrently served as the chief of the Chinese delegation to the League of Nations when it was holding its sessions in Geneva. When Lord Lytton finally presented his report to the League in February 1933 concluding that Japan had indeed committed aggression, it was the subject of much debate within the League. In March 1933, Koo gave what was described as a very strong speech, urging the League to finally act against Japan, now that it was established by the Lytton commission that Japan had committed aggression. The Japanese delegate to the League, Yōsuke Matsuoka, promptly announced that his government did not accept the Lytton report and was leaving the League, effective immediately. During the Abyssinia Crisis caused by the Italian invasion of Ethiopia in 1935, Koo was a strong supporter of having the League of Nations apply sanctions against Italy, believing this was a hopeful precedent that might led to the nations that belonged to the League supporting China should Japan invade. In May 1936, the Italians took the Ethiopian capital of Addis Ababa and Benito Mussolini proclaimed victory in Rome. Though the Italians never fully controlled the Ethiopian countryside, the conquest of Ethiopia was a major blow to the prestige of the League of Nations. Koo wrote at the time that at the League's headquarters in Geneva there was "a general sickening atmosphere at the impotence and cowardice" of the League in face of the Italian aggression against Ethiopia. On a more hopeful note for Koo, he met the Soviet foreign commissar