In Harry Turtledove's Worldwar series, the Empire of Japan is the only one of the "Big Five", or the major nations of OTL's Allies and Axis, combatting the invasion of the Race who lacks a POV character, aside from the imprisonment of a Race killercraft pilot and tangential references from Chinese POV characters. Turtledove, when asked about it, admitted that it was because he knew little of Imperial Japan at the time, something he tried to remedy later with his Days of Infamy series.
I've thought about the effects of the events of the Worldwar series and OTL and how things would play out for Imperial Japan in that ATL. The primary mover of events, and a possible POV character, I would think would be Isoroku Yamamoto, the Commander-in-Chief of the Combined Fleet, and mastermind behind the strike at Pearl Harbor. He studied at Harvard University, had two postings as naval attaché in Washington, DC, and spoke fluent English. He opposed the invasions of Manchuria and China, apologized personally to the US ambassador for the USS Panay incident, and also opposed the Tripartite Pact with Germany and Italy. He was an old opponent of the militarists in the Army and government, including wartime Prime Minister Hideki Tojo, and received a lot of hate mail and death threats, but he remained very popular with the Navy and had close relations with the imperial family. He opposed war with the United States, but planned it for the best chance at victory when the government was determined with its course to pursue it.
With the invasion of the Race on May 30, 1942 cutting off the Battle of Midway before it could begin, the world would politically become a far more comfortable place for Yamamoto with the Allies and Axis suddenly thrown into the same side against the alien invasion. One of the early actions mentioned are aircraft carriers and battleships attacking Race starships landed near the Chinese coast, presumably Japanese, that lead the Race to immediately start targeting capital ships around the world; a later observation notes that the US Navy had dispersed their Atlantic vessels throughout the ports of the eastern seaboard to prevent the Race from doing too much damage. Meanwhile, the Imperial Japanese Army is quickly overrun in occupied China, while they put up a more stout defense with the Kwantung Army in their puppet Manchukuo, though its alluded that this won't last too much longer in the face of the Race's technological superiority compared to the sparse mechanization of the IJA, and with the fall of Harbin in late 1942.
Yamamoto was very much a forward thinking officer, changing his specialization from gunnery to naval aviation in 1924, as well as opposing the building of the battleships Yamato and Musashi as a waste of resources. In OTL, he also started a project that would end up benefiting Japan in this ATL. Right after Pearl Harbor, he conceived the idea of taking the war to the American mainland via submarine-launched aircraft that resulted in a proposal in mid-January 1942 - the Sentoku type submarine, or the I-400-class submarine. In OTL, 18 were planned but construction was slow and, after Yamamoto was assassinated by the US Army, reduced to just 5, of which only 3 were completed by the end of the war. With surface capital ships being destroyed by the Race, the I-400 would become the IJN's ace-in-the-hole against them for strikes against coastal positions and would likely see construction sped up as new surface carriers and battleships are canceled. When the Race later starts targeting merchant shipping, the I-400s would also be ideal cargo submarines for transporting cargo and people between the human allies.
Also in OTL, the Japanese military attaché in Germany witnessed the trials of the Messerschmitt Me 262 in 1942 and the IJN ordered a similar aircraft, but progress was slow because they had little to go on besides photographs and a cutaway drawing of the BMW 003 turbojet engine. The Nakajima Kikka only had its first test flight the day after Hiroshima was bombed, far too late to have any impact. In this ATL, however, the Allies and Axis are all actively exchanging technology to help each other better resist the Race - the US giving their bazooka anti-tank rocket to Germany in exchange for their long range rocket tech, Britain giving radar tech to the USSR, etc. With them all on the same side, it's quite probable that Germany would be able to send far more complete diagrams and such to Japan, especially without the Allies trying to interdict such exchanges. With Race killercraft regularly bombing the Home Islands, the IJN under Yamamoto - who presumably wouldn't be assassinated in this ATL - would readily pursue a jet aircraft to try and combat them as Germany and Britain do. As Germany pursues its more advanced panzers and jets more quickly in this TL to try and catch up to the Race, I could see the Kikka beginning development in 1942 and beginning flights in 1944 in time for the final stages of the war against the Race.
Post-war, the IJN and Yamamoto's positions would be far stronger than the Army's. The militarists of the Army, embodied in the Kwantung Army that conquered Manchuria without government approval in the 1930s, opposed technological development in place of pursuing bushido, the way of the warrior and fighting spirit. That Kwantung Army has now been destroyed in Manchukuo and Korea as the Race overran them, and the IJA liaisons with Japan's atomic bomb project fouled up their handling of their Race prisoner to the extent that Tokyo got destroyed by an alien nuke. Meanwhile, the IJN will have developed the first jet aircraft to try and fight the Race's killercraft, and their I-400s would be immune to Race attacks while launching their own attacks and transporting cargo and personnel. The Army would likely realize their mistake and try to catch up technologically, but the Navy under Yamamoto has a large head start. I wouldn't be surprised if the Japanese atomic bomb project that finally made Imperial Japan a nuclear power in the 1960s was under the auspices of the IJN, as well as any space program.
I've thought about the effects of the events of the Worldwar series and OTL and how things would play out for Imperial Japan in that ATL. The primary mover of events, and a possible POV character, I would think would be Isoroku Yamamoto, the Commander-in-Chief of the Combined Fleet, and mastermind behind the strike at Pearl Harbor. He studied at Harvard University, had two postings as naval attaché in Washington, DC, and spoke fluent English. He opposed the invasions of Manchuria and China, apologized personally to the US ambassador for the USS Panay incident, and also opposed the Tripartite Pact with Germany and Italy. He was an old opponent of the militarists in the Army and government, including wartime Prime Minister Hideki Tojo, and received a lot of hate mail and death threats, but he remained very popular with the Navy and had close relations with the imperial family. He opposed war with the United States, but planned it for the best chance at victory when the government was determined with its course to pursue it.
With the invasion of the Race on May 30, 1942 cutting off the Battle of Midway before it could begin, the world would politically become a far more comfortable place for Yamamoto with the Allies and Axis suddenly thrown into the same side against the alien invasion. One of the early actions mentioned are aircraft carriers and battleships attacking Race starships landed near the Chinese coast, presumably Japanese, that lead the Race to immediately start targeting capital ships around the world; a later observation notes that the US Navy had dispersed their Atlantic vessels throughout the ports of the eastern seaboard to prevent the Race from doing too much damage. Meanwhile, the Imperial Japanese Army is quickly overrun in occupied China, while they put up a more stout defense with the Kwantung Army in their puppet Manchukuo, though its alluded that this won't last too much longer in the face of the Race's technological superiority compared to the sparse mechanization of the IJA, and with the fall of Harbin in late 1942.
Yamamoto was very much a forward thinking officer, changing his specialization from gunnery to naval aviation in 1924, as well as opposing the building of the battleships Yamato and Musashi as a waste of resources. In OTL, he also started a project that would end up benefiting Japan in this ATL. Right after Pearl Harbor, he conceived the idea of taking the war to the American mainland via submarine-launched aircraft that resulted in a proposal in mid-January 1942 - the Sentoku type submarine, or the I-400-class submarine. In OTL, 18 were planned but construction was slow and, after Yamamoto was assassinated by the US Army, reduced to just 5, of which only 3 were completed by the end of the war. With surface capital ships being destroyed by the Race, the I-400 would become the IJN's ace-in-the-hole against them for strikes against coastal positions and would likely see construction sped up as new surface carriers and battleships are canceled. When the Race later starts targeting merchant shipping, the I-400s would also be ideal cargo submarines for transporting cargo and people between the human allies.
Also in OTL, the Japanese military attaché in Germany witnessed the trials of the Messerschmitt Me 262 in 1942 and the IJN ordered a similar aircraft, but progress was slow because they had little to go on besides photographs and a cutaway drawing of the BMW 003 turbojet engine. The Nakajima Kikka only had its first test flight the day after Hiroshima was bombed, far too late to have any impact. In this ATL, however, the Allies and Axis are all actively exchanging technology to help each other better resist the Race - the US giving their bazooka anti-tank rocket to Germany in exchange for their long range rocket tech, Britain giving radar tech to the USSR, etc. With them all on the same side, it's quite probable that Germany would be able to send far more complete diagrams and such to Japan, especially without the Allies trying to interdict such exchanges. With Race killercraft regularly bombing the Home Islands, the IJN under Yamamoto - who presumably wouldn't be assassinated in this ATL - would readily pursue a jet aircraft to try and combat them as Germany and Britain do. As Germany pursues its more advanced panzers and jets more quickly in this TL to try and catch up to the Race, I could see the Kikka beginning development in 1942 and beginning flights in 1944 in time for the final stages of the war against the Race.
Post-war, the IJN and Yamamoto's positions would be far stronger than the Army's. The militarists of the Army, embodied in the Kwantung Army that conquered Manchuria without government approval in the 1930s, opposed technological development in place of pursuing bushido, the way of the warrior and fighting spirit. That Kwantung Army has now been destroyed in Manchukuo and Korea as the Race overran them, and the IJA liaisons with Japan's atomic bomb project fouled up their handling of their Race prisoner to the extent that Tokyo got destroyed by an alien nuke. Meanwhile, the IJN will have developed the first jet aircraft to try and fight the Race's killercraft, and their I-400s would be immune to Race attacks while launching their own attacks and transporting cargo and personnel. The Army would likely realize their mistake and try to catch up technologically, but the Navy under Yamamoto has a large head start. I wouldn't be surprised if the Japanese atomic bomb project that finally made Imperial Japan a nuclear power in the 1960s was under the auspices of the IJN, as well as any space program.