But since I'm no expert on the AJ-Alliance, I'll give the MCDO SIGNAL.
Lol. Well, I certainly can't ignore that.
Before I start, I'll just say two things: One, I am not an expert on anything, much less the AJA. Two, I have given fairly long/detailed posts before on what (IMHO) caused the rise of the Militarists, and how it could have been avoided. I am happy to go into more detail here later, if asked; but for now I will try to keep it short.
First, on the Anglo-Japanese Alliance. In OTL it fell apart for two main reasons. The first reason is American pressure. By the early 1920s the US was already worried about a potential conflict with Japan, and so they were against any other great power maintaining an alliance with Japan. (There are lots of reasons for the tension between the US and Japan: the US wanted to maintain the "open door" in China, which the 21 Demands seemed to threaten; a lot of nationalist feeling had been raised/angered in Japan over the well-publicized poor treatment of Japanese immigrants in the US, including a ban on Japanese ever naturalizing in the US; general tension that comes from having two rapidly-growing naval powers bordering the same ocean, etc. But I won't go into them here.) Obviously in OTL, the UK decided better ties with the US were worth more than an alliance with Japan. However, even in OTL, there were people in high places arguing against this decision. They argued that an
alliance with the US was preferable to an alliance with Japan, but no such alliance was being offered. Instead, they were trading an alliance that had just been honored in wartime for general good feeling in a US that had just retreated into isolationism and refused to join the League that Wilson had pushed so hard for. So, with the right PoD, I think it is possible for the UK to choose to maintain the alliance even over US displeasure. As for Japan, I think the more self-confident Japan of 1920 will be willing to work less hard/give up less to maintain the alliance than their fathers were in 1902. However, if the alliance is made somewhat closer to one of equals, I think they will be happy to maintain it. There were certainly no shortage of very high-ranking admirals even in the Militarist period who abhorred the idea of war with the US/UK, and wanted to restore the alliance and better ties.
The second reason the alliance was ended was more simple: they ran out of enemies. The original alliance was aimed at Russia: Japan was afraid of Russian designs on Manchuria and Korea, and Britain was worried about possible Russian expansion in Central/South Asia. Taking this with the first issue, I think the easiest way to keep the alliance going is to change WW1. It really is unusual, I think, that WW1 saw the destruction of both Germany
and Russia--who would have predicted that in 1914? If either of them survives the war, or indeed comes out of it stronger (instead of being ruined for a generation as a power), then the alliance will still have its raison d'etre. Let's imagine, for the sake of argument, a TL in which the Haber process is not invented in Germany in time. The war starts on schedule, but the blockade bites, Germany runs out of munitions, and the war collapses. In this scenario, it's easy to imagine that Russia comes out of the short war stronger (indeed, Germany too will not be ruined), and where in the post-war British and Japanese fears of Russia resurface. In a short war, Japan (or factions therein) will likely not push the 21 Demands (as there is no impression that Europe is distracted and will be for some time), which leads to better relations between Japan and the US and Britain. Finally, if the war is short, the US will never become involved. They will still be something of an untested military power, and people will not be certain if they would choose to become involved in a future conflict between great powers. Therefore, in that TL I think it's likely the AJA would continue. That's not the only way to make this happen, of course, but I think it is one way that would work.
OK, so (briefly) ways to help butterfly/prevent the rise of the Militarists generally:
- in 1894 (First Sino-Japanese War), the army tried to ban the Prime Minister from attending meetings at Imperial General HQ. Emperor Meiji stepped in and forced them to admit him. In 1904 (Russo-Japanese War), they again banned the PM from attending. This time (quite possibly because Emperor Meiji was older and less healthy) it stuck. If this is reversed, it will help set a precedent that the civilian government does have a role to play in the planning and management of wars; that it is not the exclusive province of the military alone.
- Prevent or reverse the outcome of the Taisho Constitutional Crisis. This was a close-run thing--originally the Privy Council sided against the military--but it eventually it established the precedent that the army and navy could collapse any government they chose. It was the fatal flaw of the Meiji Constitution, and more than anything else prevented democracy from truly working. Obviously, preventing that alone would be huge.
- Do something about rural poverty. In an age prior to birth control, rural families often had many children, and worked tiny unproductive plots. They lived lives of crushing poverty, and this only got worse during the Great Depression, when real rural income declined by half. All of this made the rural youth easy targets for radicalization. If something can be done about this, there will be far less radicalism in the army, and less support for radical ideas generally.
- Reduce the size of the army. Throughout the 1920s, there was a conflict over what shape the army of the future should take. On one side were those who believed that any war that Japan was likely to enter would be over quickly. They believed that such a war would be decided within a few short months to possibly a year or two, and therefore Japan had to maintain a large standing army at all times, in order to quickly win a decisive victory. On the other side were those who believed that Japan's next war would probably be a long conflict (a la WW1). Those who plumped for a multi-year conflict wanted to reduce the size of the army in order to save money, and pump that money into industry, technology, etc. Then when war came, they would have a smaller army, but at a much higher standard, and the army could be rapidly expanded around this core. There are lots of reasons why the "short war, big army" side won--not least, I think, because many in the army wanted a big force for political reasons and many civilians favored a big army as a way to vacuum up otherwise unemployed young men. At any rate, if the "long war, small army" side wins, that will help reduce the influence of the Militarists simply because fewer young men will enter the army and be exposed to these kinds of beliefs.
- Improve the economy. It is no coincidence that Taisho Democracy (and a relatively benign foreign/colonial policy) took place at the same time as rapid economic growth. But in 1927, a banking crisis (the Showa Financial Crisis) occurred, followed by the Great Depression. The Great Depression didn't hurt Japan too much, I would argue, but the Smoot-Hawley Tariff and the resulting massive (50-66%) drop in world trade did. Just the same as in Germany or a dozen other countries, reduce economic instability and you will greatly reduce political instability as well.