The French continued to pressure Siam, and in 1906–1907 they manufactured another crisis. This time Siam had to concede French control of territory on the west bank of the Mekong opposite Luang Prabang and around Champasak in southern Laos, as well as western Cambodia. France also occupied the western part of Chantaburi. In the Franco-Siamese Convention of 13 February 1904, in order to get back Chantaburi, Siam had to cede Trat to French Indochina. Trat became part of Thailand again on 23 March 1907 in exchange for areas east of the Mekong river including Battambang, Siam Nakhon, and Sisophon. The British interceded to prevent more French expansion against Siam, but their price, in 1909 was the acceptance of British sovereignty over Kedah, Kelantan, Perlis and Terengganu, under the Anglo-Siamese Treaty of 1909. All of these ceded territories were on the fringes of the Siamese sphere of influence and had never been securely under Siamese control, but being compelled to abandon all claim to them was a substantial humiliation to both king and country. In the early 20th century these crises were adopted by the increasingly nationalist government as symbols of the need for the country to assert itself against the West and its neighbours. Shortly before World War II, the French government agreed to border negotiations with Thailand which were expected to make minor changes in Thailand's favour. However, France soon fell to Hitler's forces, and the negotiations never took place. Thailand then took advantage of French weaknesses to reclaim its lost territories in French Indo-China, resulting in the French-Thai War between October 1940 and 9 May 1941. Thai military forces did well on the ground and in the air to defeat the French and regain her territory, but Thai objectives in the war were limited. In January, however, Vichy French naval forces achieved a tactical victory over Thai naval forces in the Battle of Koh Chang without much loses on both sides. The war ended in May with the help of the Japanese, allied with Nazi Germany, who coerced the French to relinquish their hold on the disputed border territories. To commemorate the victory, Thailand erected the Victory Monument. Thailand invited Japan and Germany to join the celebration. Japan sent Shōjirō Iida, and the German Foreign Ministry sent Robert Eyssen, to join the celebration. After the war, in October 1946, northwestern Cambodia and the two Lao enclaves on the Thai side of the Mekong River were returned to French sovereignty after the French provisional government threatened to veto Thailand's membership in the United Nations.