Typing on my phone at the moment, so please forgive any typos/syntax errors. Furthermore first post in this sub but I feel it falls within my field of expertise.

Did Shinto influence nationalism? Most definitely so. After the Meiji revolution, the government became highly centralised with the emperor at its core. Buddhism became sidelined and Shintoism was heavily promoted in schools. The emperor was in effect the traditional head of the Shinto faith, and as such the religion was used to validate his divine authority. Compared to most pre-industrialised countries of the era, Japan already had a fairly developed education system and a high literacy rate and as such was able to indoctrinate many people to the new political creed in a relatively short space of time. The conflict between Russia and Japan is the earliest example of the influence of Shinto in the Japanese military. Yasukuni shrine (famous for housing the remains of convicted of ww2 war criminals) was first used to during the 1904 conflict to venerate the fallen soldiers. Every day, the emperor himself would sign a tapestry containing the names of fallen soldiers and acknowledge the sacrifice they had made in his name and grant them rest in the afterlife (on display at the Yasukuni 'museum' ; full of propaganda so take it with a pinch of salt)

Did Shintoism turn into a political ideology? Perhaps not an ideology in its own right, but it definitely served to support the imperial/military authority of the 1930s. Soldiers served in the name of the emperor and ultimately obeyed his whim (for example, coup d'etat of 1936 carried out in the name of the emperor ultimately failed because he hadn't given it his backing). Keep in mind that at its core, the Shinto faith is largely unstructured and myth based (comparable to ancient Greek and Norse religions). Therefore it is hard to use it for ideological purposes in the same way as Abraham religions. However it may have influenced the idea of Japanese racial superiority: that Japan is unique compared to the rest of the world.

Was it responsible for ww2? Probably not. But it did influence the way Japanese fighters chose to fight. Japanese fighters were relentless despite being severely underequipped and would often fight against hopeless odds. Kamikaze (literally divine wind) are a good example of soldiers dying in vain for the emperor (so the legend says, there were always more volunteers for Kamikaze attacks than there were planes). Hope that helps, but it's 2am over here so I'm going to bed. Main sources for this are Japan and the shackles of the past by Taggart Murphy and the making of modern Japan by marius Jansen (I would highly recommend the latter for anyone interested in the 1700-1980s period of Japanese history). I'd also recommend visiting Yasukuni shrine museum if you ever have the time for a more in depth look, but keep in mind that there's a lot of propaganda there)

Hmm, I thought I had written something about this, but it looks like I didn't. Sorry about that.

I'm not the best expert on the development of modern Shinto as a whole, but roughly speaking Shintoism is really just a unifying name for the synthesized relationship between Japanese folk religion and Buddhism that developed over the years. There are a few key ideas that I will specifically focus on, namely:

a) The Japanese and/or the Emperor being descended from the gods

b) The existence of "kami"

c) The implications about the body and soul

So while the idea of establishing Shinto as a "state" religion was rejected by the Diet, it nonetheless remained a part of the modern Japanese identity. It was thought that the Japanese were descended from the gods themselves, and that the Emperor was also directly descended from the sun goddess Amaterasu. This in a sense is reflective of both the idea of "American exceptionalism" as well as the "divine right of kings/Mandate of Heaven." In a sense, the idea was that to be Japanese was to be special, to be divine. This served as a useful propaganda tool in the war, but it wasn't specifically created for that purpose. Rather, the initial idea comes from a literal interpretation of the Japanese mythological origin story, where the gods created the Japanese people.

So the second things is about the "kami" which are usually translated as "Gods," which is a perhaps imprecise translation. It doesn't help that the Christian "God" is translated as "kami" as well. The Japanese kami really is more like a type of spirit that inhabits objects and buildings, and can control their nature. For instance, the infamous Yasukuni Shrine is inhabited by a kami that is composed of the spirits of those who had died in battle for the Empire of Japan. By comparison, Meiji Shrine is inhabited by the kami of the Emperor Meiji and his wife, and various other shrines throughout the country are inhabited by whatever the local folklore is. The idea of the "kamikaze" thus is not the idea that the gods personally intervened to save Japan from the Mongols, but rather the kami of the winds were doing their duty to defend Japan from invaders (and in a sense, that is what the kamikaze squadrons were supposedly replicating).

Rather than saying that Shinto was integrated into Japanese nationalism, I would say that Japanese nationalism had always incorporated these sorts of religious points to begin with, which just happened to be isolated as so-called State Shinto.

From what I can tell Shintoism was not involved in the Japanese decision to go to war. For instance, for the Combined Fleet review prior to the Pearl Harbor attack, the reasons given as to the Japanese attack on Pearl Harbor did not mention any sort of religious motivation. Neither was it involved to go to war against the Chinese-rather, both the Chinese and the Japanese viewed the war they were fighting as a defensive war, due to the propaganda on both sides (although it was mostly the Japanese military's fault).

However, Shinto ideas were certainly used as propaganda to encourage the troops to fight harder and not surrender. Buddhist principles incorporated into Shintoism preached for example the integrity of the body. As such, Japanese soldiers were regaled with tales of how Allied troops would mutilate Japanese soldiers that surrendered. This was far more horrifying a thought for them than for us-imagine it being the equivalent of being told that if you surrender you will irrevocably go to hell. Add the fact that the Allies did in fact take a lot of war trophies (skulls, bones, and such) and the Japanese had an extremely effective propaganda machine of getting their troops to fight to the death.

Sources:

de Bary, Sources of Japanese Tradition

Ienaga, The Pacific War

Dower, War Without Mercy

Mason, A History of Japan

In response to your first question regarding Shinto/nationalism:

• See Fujitani Takashi's Splendid Monarchy where it lays out how the Meiji leaders shaped the nation through the manipulation of Shinto practices and superstition and tying these ideas into the created symbol of the Emperor.

• Shinto before 1868 was highly merged with Buddhism, but in the Meiji period Shinto and Buddhism were separated. Shinto, as it is more a vague space of terms, became an easy vehicle to create the concept of a nation and regain public confidence since there was a regime change and the populace was not too pleased with its government's cozy tendencies with the foreigners barraging into Japan.

• How was Shinto used? It gave a mythical element to the Emperor and legitimacy. It made the country "united" from a random set of villages that are mildly aware of each other. The leadership was highly involved in this process, and many aspects were manufactured. This is where some "traditional Japanese" statements get said in public discourse, when in actuality, it's made up traditional Japanese.

• Yes, technically Shinto became a bit of a political ideology, generally just known as State Shinto, since it was run by the government. (It even has its own branch for a few years.)

• I wouldn't say Shinto influenced the decision to participate. Japan's decisions as early as 1931 were clearly not Shinto/religious based, but more so colonial and imperial. However, sometimes it was used as part of political rhetoric to spearhead causes or excuse them.

• Lastly, background speaking: for a good understanding of the Shinto/Buddhism divide that happens with the Meiji Restoration, I highly recommend The Invention of Religion in Japan by Jason Josephson.

So basically, tl;dr: Shinto is used and abused by leadership, because by removing Buddhism from Shintoism it's a far more broad, open-ended idea of the world than a strict religion. Therefore, it's easy to use and manipulate in the political sphere.

One could easily say that rising nationalism influenced and manipulated Shinto. Japan falsely attributed Western powers' strength to having a Christian justification for its imperial adventures. It sought to mimick that by separating Shinto from Buddishm (they were previously not very clearly demarcated in the popular mind) and controlling Shinto under the state. As a side note, not all Shrines allowed themselves to be taken over.

With State Shinto established, Imperial Japan then promoted the idea that war dead will be commemorated in Yasukuni Shrine. Shinto was used to justify the Emperor as being descended from [Amaterasu Omikami] (http://en.wikipedia.org/wiki/Amaterasu), the sun goddess, and in turn Japan's special closeness to the gods (the phrase "kami no kuni" -- god's country) being used to justify its rightful imperial status in Asia.

Originally Shinto was an animist religion mostly concerned with nature worship -- mountains, rivers, trees etc being "kami" -- and ancestor worship. It has little in the way of political prescriptions. Imperial Japan clearly took it over and used it for political ends, parroting the way Western powers used Christianity as a rationale for imperialism.

Edit: formatting

Westerners have some misunderstandings with Shinto during WW2. There was a prime minister of Christian before the WW2 in Japan.