Aritomo Yamagata (1838 - 1922) was born of samurai stock in the Chosu Clan, but was the key figure, in the Meiji restoration, of ousting the samurai from political power. He was the principal architect of the Imperial Japanese Army. While he died long before World War Two in the Pacific, he shaped the Army that would fight in it. His thinking affected the geopolitical doctrine of Karl Haushofer, who studied in Japan.
In 1864, he was among the first members of the mixed rifle units (shotai) formed by Shinsaku Takasugi of the Chosu Clan, the first non-samurai combat units. Initially, he saw the shotai, or the kiheiti "shock troops" under which they were organized, as a weapon against militarily superior foreigners. Increasingly, however, he became opposed to the Tokugawa Shogunate, becoming a member of the Choshu shishi ("men of spirit") who wanted to restore Emperor rule and break the shogunate. 
Building Meiji's Army
Whe the Franco-Prussian War broke out in 1870, Yamagata was in Europe, as one of his Chosu proteges, Taro Katsura. Both studied, and were impressed by German methods. Previously, Masujiro Omura, who had been an architect of the Chosu Army, had emphasized French training used by the Shogunate's officers. Omura was assassinated in 1869 by a Chosu samurai, angry that this model allowed for non-samurai soldiers.
Yamagata was concerned with spirit, training, and manpower. In 1872, conscription was introduced. With the assistance of a French mission, also in 1872, an education system was started, beginning with the Toyoma School, replacing an Omura organization, for noncommissioned officers. The Military Academy (Japan) was formed in 1875, modeled on France's St. Cyr.
While he worked to adapt bushido to mass armies, it would take ten years before the Imperial Rescript to Soldiers and Sailors would be issued in 1882. It had changed from the original emphasis of obedience to senior officers and changed to absolute loyalty to the Emperor, which may have helped establish the junior officer structured insubordination of gekokoju.
He was Chief of Staff three times: (24 Dec 1878 - 4 Sep 1882), (13 Feb 1884 - 22 Dec 1885), (20 Jun 1904 - 20 Dec 1905)
Entry to government
After serving as "taifu (senior vice minister) of Army-Navy Ministry, and army taifu, Yamagata became war minister in 1873. Promoting conscription, he focused on building the military system." He became Chief of Staff (Imperial Japanese Army) in 1878, Home Minister in 1883, and home minister in the first Ito cabinet in 1885.
He was not a democrat, and indeed suppressed the people's rights movement, focusing on building a centralized system that controlled local government. "In 1889, he formed his first cabinet, and successively held important posts including justice minister, war minister in the second Ito cabinet, chairman of the Privy Council.
First Sino-Japanese War
He commanded the First Army during the First Sino-Japanese War, 1894–95.
In 1898, he formed his second cabinet.
During the Russo-Japanese War, he commanded operations as chief of the General Staff.
World War I
He cautioned Emperor Taisho against early involvement in the Siberian Intervention, although agreed to move when Britain sent a ship to Vladivostok, and, in 1918, when U.S. initiatives started, saying, on July 12, "Since the present expedition is not to fight with Germany, there is no need for our inadequacy to give anxiety." 
As genro (elder statesman), who recommended candidates for Prime Minister of Japan, he effectively gathered government officials and military men, into what was called the "Yamagata-batsu (clique)," and exerted major influence on the political establishment."
As for Prince Yamagata, it is incumbent upon him to resign all the public office he holds, to say nothing of renouncing his treatment as a genro, so as to apologize to the emperor and to the nation. Otherwise, it would be impossible to placate the nation, which feels high resentment against his attitude. 
- Merion and Susie Harris (1991), Soldiers of the Sun: the Rise and Fall of the Imperial Japanese Army, Random House, pp. 12-14
- Harris & Harris, 20-24
- David Bergamini (1971), Japan's Imperial Conspiracy, Morrow, p. 304
- Yamagata, Aritomo, National Diet Library
- Sterling Seagrave and Peggy Seagrave (1999), The Yamato Dynasty: the secret history of Japan's imperial family, Broadway Books, ISBN 07677904066, p. 155