its 1905 victory in the Russo-Japanese War, as Phan Bội Châu encouraged Vietnamese youth to travel to Japan and study in preparation for revolution against the French colonial government in what was called the Đông Du movement; among these students was Cường Để, heir to the throne of the Nguyễn dynasty. The Russo-Japanese War had created diplomatic tension between France and Japan due to France's closeness to Russia throughout the conflict, leading to the Franco-Japanese Treaty of 1907. The treaty improved relations between Japan and French Indochina, prompting Japan to crackdown on Vietnamese students in the Đong Du movement who by 1910 had either fled Japan or been deported, including Cường Để who, like Phan Bôi Châu, escaped into self-imposed exile. Japanese women called Karayuki-san migrated to cities like Hanoi, Haiphong and Saigon in colonial French Indochina in the late 19th century to work as prostitutes and provide sexual services to French soldiers who were occupying Vietnam since the French viewed Japanese women as clean they were highly popular. Images of the Japanese prostitutes in Vietnam were put on French postcards by French photographers. The Japanese government tried to hide the existences of these Japanese prostitutes who went abroad and do not mention them in books on history. During the 19th century the Đại Việt sử ký toàn thư was distributed in Japan. In 1883 Toshiaki Hikida, a military officer of the General Staff Office of the Japanese Imperial Army, was assigned his post in Vietnam, during his stay there a local mandarin in Hanoi would give him a copy of the Đại Việt sử ký toàn thư which after returning to Japan in 1884 he would publish and reprint there. Afterward, Hikida's version of the Đại Việt sử ký toàn thư would become widely circulated outside of Vietnam. World War I On 27 August 1914 Japan officially entered the World War I on the side of the allies (also known as the Entente Powers), Japan invaded and took the German colony of Tsingtao and the rest of the Kiautschou Bay Leased Territory. In November 1914 Japan would supplant the German sphere of influence in southern China with its own political and economic influence making putting it in direct competition with French Indochina. Even though the Japanese openly supported a number of anti-French secessionist movements, such as Prince Cường Để's Duy Tân Hội, the French situation in Europe was bad