Short answers, pulled from the story.
The Empire of Japan began on the 3rd of January 1868 when Crown Prince Mutsuhito ascended the throne as Emperor Meiji. The Empire officially ceased to exist on the 3rd of May 1947 when the new Constitution of Japan took effect.
Emperor Meiji, born Crown Prince Mutsuhito, led the Empire of Japan starting from his ascension on the 3rd of January 1868. He oversaw the violent end of the Tokugawa shogunate and the subsequent modernization of the nation.
The Empire of Japan joined the Axis powers on the 27th of September 1940 by signing the Tripartite Pact with Germany and Italy. This decision was driven by the need to secure oil and raw materials after the United States imposed embargoes.
The Empire of Japan formally surrendered on the 2nd of September 1945 following the atomic bombings of Hiroshima and Nagasaki. This surrender occurred in compliance with the Potsdam Declaration and the Soviet invasion of Manchuria.
The Meiji government recruited over 3,000 Western experts known as O-yatoi gaikokujin to teach modern science and mathematics. These experts were hired to help transform Japan from a feudal society into a global industrial power.