As the name suggests, the nation was an imperial power that unified the area of Japan, and ruled the region until the government was removed at the end of World War II. Imperial Japan fell after expansion led Japan to join the Axis Powers in opposition to an alliance of nations that included the United States of America.
The Imperial Japanese were known to have severely provoked conflict with a surprise bombing of Pearl Harbor, a preemptive strike remembered as one of history's tragedies. (TNG episode: "The Enemy"; DS9 episode & novelization: Far Beyond the Stars)
The Imperial Japanese military included a sizable fighting force of soldiers, sailors and pilots. Among those in the service of the Empire was Second Lieutenant Nogami, an army officer who was kidnapped by the Briori in the year 1937. (VOY episode: "The 37's")
Many notable ships were deployed in the service of the empire. Numerous vessels were succeeded in name by future vessels, including many starships. Some of these storied names include Akagi, Fuso, Haruna, Hiryu, Chikuma, Kaga, Kashima, Kitakami, Kongo, Maya, Musashi, Nagato, Ryujo, Soryu, Tsushima, Yamashiro, Yamato, Zuiho and Zuikaku.
Technology and weapons
In an alternate timeline where the Na'kuhl interfered with the normal flow of history prior to World War II, the Nazis were given a tactical edge due to futuristic technology. The Na'kuhl directly assisted another Axis power, the Nazis, and as such, caused the United States to not be able to effectively combat attacks from either front of the war. In this reality, the Imperial Japanese were able to devastate the US Navy, including the sinking of the USS Enterprise aircraft carrier. This reality was averted when Jonathan Archer and the crew of the Enterprise NX-01 intervened, and assaulted a time-changing device the Na'kuhl were planning to use to complete their disruption of history. (ENT episode: "Storm Front")
The City on the Edge of Forever
After traveling through the Guardian of Forever in 2267 to New York City, 1930, Leonard McCoy saved Edith Keeler's life, an act which, unbeknownst to him, altered Earth history. McCoy spent years in the 20th century, expecting his colleagues to follow him back in time and rescue him. McCoy was surprised when December 7, 1941 passed without news of the bombing of Pearl Harbor. He realized then that, because Edith Keeler had survived her brush with death years earlier, she was able to become a powerful voice in the American Pacifist Movement and exert significant influence on the Roosevelt Administration, so much so as to delay or prevent the United States entry into World War II. By 1944, Imperial Japan had conquered Australia and New Zealand and finally struck against the United States in Hawaii and drew the nation into war, which ended up being won by Adolf Hitler's Nazis prevailing over all with the atomic bomb. (TOS episode & Star Trek 2 novelization: The City on the Edge of Forever; TOS - Crucible novel: Provenance of Shadows)