See R. F. Hackett, Yamagata Aritomo in the Rise of Modern Japan, 1838-1922 (1971).
After the defeat of the Tokugawa, Yamagata together with Saigō Tsugumichi was selected by the leaders of the new government to go to Europe in 1869 to research European military systems. Yamagata like many Japanese was strongly influenced by the recent striking success of Prussia in transforming itself from an agricultural state to a leading modern industrial and military power. He accepted Prussian political ideas, which favored military expansion abroad and authoritarian government at home. On returning he was asked to organize a national army for Japan, and he became War Minister in 1873. Yamagata energetically modernized the fledgling Imperial Japanese Army, and modeled it after the Prussian army. He began a system of military conscription in 1873.
Yamagata in 1877 led the newly modernized Imperial Army against the Satsuma Rebellion led by his former comrade in revolution, Saigō Takamori of Satsuma. At the end of the war, when Saigo's severed head was brought to Yamagata, he ordered it washed, and held the head in his arms as he pronounced a meditation on the fallen hero.
He also had Emperor Meiji write the Imperial Rescript to Soldiers and Sailors, in 1882. This document was considered the moral core of the Japanese army and naval forces until their dissolution in 1945.
Yamagata was awarded the rank of field marshal in 1898. He showed his leadership on military issues as acting War Minister and Commanding General during the First Sino-Japanese War; as the Commanding General of the IJA 1st Army during the Russo-Japanese War; and as the Chief of the General Staff Office in Tokyo.
Yamagata and Itō Hirobumi were long the most prominent of the seven, and after the assassination of Itō in 1909, Yamagata dominated the genrō. But Yamagata also held a large and devoted power base in the officers of the army and the militarists. He became the towering leader of Japanese conservatives. He profoundly distrusted all democratic institutions, and he devoted the later part of his life to building and defending the power, especially the political power, of the army.
During his long and versatile career, Yamagata held numerous important governmental posts. In 1882, he became president of the Board of Legislation (Sanjiin) and as Home Minister (1883–87) he worked vigorously to suppress political parties and repress agitation in the labor and agrarian movements. He also organized a system of local administration, based on a prefecture-county-city structure which is still in use in Japan today. In 1883 Yamagata was appointed to the post of Lord Chancellor, the highest bureaucratic position in the government system before the Meiji Constitution of 1889.
Yamagata became the third Prime Minister of Japan after the opening of the Imperial Diet under the Meiji Constitution from 24 December 1889 to 6 May 1891. During his first term, the Imperial Rescript on Education was issued.
Yamagata became Prime Minister for a second term from 8 November 1898 to 19 October 1900. In 1900, while in his second term as Prime Minister, he ruled that only an active military officer could serve as War Minister or Navy Minister, a rule that gave the military control over the formation of any future cabinet. He also enacted laws preventing political party members from holding any key posts in the bureaucracy.
He was President of the Privy Council from 1893-94 and 1905-22.
From 1900 to 1909, Yamagata opposed Itō Hirobumi, leader of the civilian party, and exercised influence through his protégé, Katsura Tarō. After the death of Itō Hirobumi in 1909, Yamagata became the most influential politician in Japan and remained so until his death in 1922, although he retired from active participation in politics after the Russo-Japanese War. However, as president of the Privy Council from 1909 to 1922, Yamagata remained the power behind the government and dictated the selection of future Prime Ministers until his death.
In 1912 Yamagata set the precedent that the army could dismiss a cabinet. A dispute with prime minister Marquis Saionji Kinmochi over the military budget became a constitutional crisis, known as the Taisho Crisis after the newly enthroned Emperor. The army minister, General Uehara, resigned when the cabinet would not grant him the budget he wanted. Saionji sought to replace him. Japanese law required that the ministers of the army and navy must be high-ranking generals and admirals on active duty (not retired). In this instance all the eligible generals at Yamagata's instigation refused to serve in the Saionji cabinet, and the cabinet was compelled to resign.