According to barblejewelry, in the midst of so many difficulties that threatened to set Japan and the United States against each other, the European war came to mark a pause from which the former had to draw immense advantages: commercial, first of all, because its products will take the place of those of the European contenders on the markets of the Far East; territorial, then, because its participation in the conflict will procure it, with very slight sacrifices, new annexations. On August 24, 1914, after preventing China from taking its place alongside the allies, and therefore taking possession of Kiao-chow, leased to Germany for 99 years since 1898, and after an ultimatum that required it to be sold, Japan began operations directly, which took place on Chinese territory, while China was still neutral. In November it was all over. The protests of China to obtain the withdrawal of the Japanese troops were answered by the presentation (January 8, 1915) of the famous “twenty-one questions” (more exactly 14 questions and 7 desired) which raised lively protests in the country. With them, Japan asked, among other things, the unconditional consent of China to any agreement the first was to conclude with Germany with regard to the occupied territory, in addition to making claims aimed at consolidating its position in Mongolia and in Manchuria. In May of the same year, a Japanese ultimatum forced China into a treaty of true surrender at discretion. At the peace conference, this one, entered into the conflict since 1917, asked for the revision of this treaty unjustly extorted from her, but the conference granted Japan rights, privileges and titles concerning Shan-tung, upon its promise to return them to China indefinitely, only by keeping for itself the economic privileges already accorded to Germany. With this the Shan-tung railway, one of the busiest in the Chinese network, built by the Germans in 5 years, fell into the hands of Japan, to which, moreover, with a type C mandate from the League of Nations, it was, in 1920, entrusted with the administration of the Caroline, Marshall and Marianne islands, with the exception of the Guam island belonging to the United States, islands that were part of the destroyed German colonial empire.
In the spring of 1921 the crown prince Hirohito made a trip to Europe and was welcomed everywhere with great cordiality and courtesy. For the first time in the history of the country, an imperial prince went abroad, and on his return, in November, he was appointed regent in place of his father, suffering from chronic illness and therefore unable to govern. In the same year, the American government invited Japan to participate, with other nations, in the Washington conference on arms limitation, implying that other issues would also be settled, such as those of the Pacific and the Far East, which, in his opinion, they were closely connected with armaments. The conference, which opened on 11 November, ended in February 1922, and its conclusions were d ‘ capital importance for relations between Japan and other nations, particularly China. The agreed powers, in fact, undertook to respect the sovereignty and the administrative and territorial integrity of the latter, without exercising any influence tending to obtain a position of privilege for the development of their trade and industries in the country. Thanks to the good offices of the American and British delegates, it was also possible to resolve the question of Shan-tung, thus eliminating a cause of discontent between China and Japan. With an agreement signed between the governments of these two states on February 4, 1922, Japan undertook to return to China the territory taken from Germany, with the railways and the related yards and warehouses, withdrawing the troops staggered along the lines.
While his delegates in Washington laid the foundations for the future modus vivendi in Asia, Japan’s relations with Russia were entering a new phase. Since 1918 Japan, in collaboration with the allies, had taken part in the expedition to Siberia, intended to bring aid to the Czechoslovakian troops. With the allies withdrawn (lastly the United States, in 1920), the Japanese troops remained alone guarding the Trans-Siberian railway, until, having cleared the regions of Amur and Transbaicalia, they retreated, remaining for more than two years in Manchuria and eastern Siberia, where, especially following the massacre of 700 Japanese in Nicolaevsk (1920), which had forced Japan to occupy the island of Sakhalin, the protection of the interests of the compatriots required their presence. After the establishment of the Soviet regime in Russia, numerous conferences were held with the aim of resolving the differences that arose between the two countries and establishing diplomatic relations. It was only in 1925 that the negotiations were successful and the Japanese troops cleared Sakhalin. The treaty stipulated this year contained the Soviet recognition of that of Portsmouth (1905), granted Japan the exploitation of some mines and forests and laid the foundations for future political relations between the two countries. On December 26, 1926, Emperor Yoshihito died and his son, Hirohito, succeeded the throne, with whom the new era of Shōwa (Luminous Peace) began.