One idea I had, inspired by Tiro's post ages back, was that during the inter-war period the Union suffers from what will ultimately be skewed priorities - because they think tey've got the South beat, they begin to look outwards and less at war on the North American continent. The reforms of the Socialist administrations also drain funding away from the military towards peace time development. As a result, the US Army shrinks and increasingly focuses on partisan warfare (from Mormons and Canadians) and less on combined arms combat. The army officer corp shrinks and a lot of its better members to other service branches - while there are still good officers, there are just aren't enough of them and the Confederate officer corp is of more even and generally superior quality. While there are efforts made to rectify this with the rise of Jake the Snake, it takes too long and thus the US Army still isn't ready, either in terms of equipment or training, on the eve of the SGW (hence still using bolt-actions when the Rebs have semi or fully auto rifles). This explains the poor state of the US Army despite US economic potential.
Conversely, the US Navy, Air Force and Marine Corp remain well funded and effective, as it is believed they will take the fight to the British or Japanese or perhaps even the Germans. In particular the USMC is the cream of the crop when it comes to US land forces - it is relatively large for an expeditionary force (though still far, far smaller than the army) and also receives more modern equipment. In particular I am thinking that the first US semi-automatic rifle (or perhaps even battle rifle) is adopted by the USMC (I am inspired by the Soviet Naval Infantry having a lot of SVT-40s at the start of WW2 and giving the Wehrmacht problems) and while they can't hope to stop the Southern tide, their better training and equipment compared to the US Army gives them a reputation as the fiercest troops in the US of A, putting up the toughest fight Johnnie Reb ever had during the first few months of the SGW. Eventually things even out more with the Army but the USMC still maintains this reputation as an "ass-kickin" force due to those early efforts.
This is also because I like the idea of the Confederate Marine Corp being underfunded and having a reputation for relative ineffectiveness (in contrast to the Confederate Army, which develops a mythos comparable to OTL Heer). So you have a army-marines contrast between the two nations.
The decision to focus military funding on fighting overseas conflicts to the detriment of a Second Round with the South would be identified by TL-191 historians a one of the Union's worst mistakes during the interwar period, as it ultimately allowed the South to gain the initial momentum during the Second Great War.
I imagined some thing similar to what you and Tiro thought about the state of the U.S. military in TL-191 after FGW: Due to Socialist policies, the strength and funding of it is nerfed down considerably that later TL-191 historians would mention that had it not been like that, the SGW would have been very different (more than likely ending in a faster victory for the USA).
It also gives some plausibility for the CSA to be able to have a decent fighting chance against the USA, although it is not necessarily a guarantee for another Southern victory.
On the earlier discussion about American stahlhelm, I have this to say in all honesty.
The US army OTL was very close to adopting the stahlhelm actually. The Nazi association led for the m1’s selection though.
Keep in mind many foreign nations favored the stahlhelm, and its design today influences modern ballistic helmets. America not the power it was otl at the start of wwii. It might lean on Germany quite a bit more ttl, so it is likely more than just the stahlhelm will be adopted. The American uniform of wwi and early wwii was very Anglo influenced and used the Brodie after all, which was discarded as the Brodie was outdated, a problem the stahlhelm wouldn’t suffer.
Northern America likely has string Germanic roots felt as a result of being allied in both wars and the immigration. Certainly the German language wouldn’t decline like otl in America. Germany’s vast colonial empire will make it a relevant language internationally.
Actually here is a good poster I found for the TL, keep in mind despite Germanic features on the American, these uniforms are not identical in reality.
View attachment 531305
German American relations will remain strong post war. Why? Because for one, America has more difficult string to deal with. Reintegrating confederacy, which likely has separatism, Mormons, and Canadians, and also japan, a far more relevant strategic threat. Japan will not he trusted by either Germany or America, after all, they only joined out of opportunism and went to the party late. Germany will also probably be terrified over the potential for a revived Russia, which is a far more real threat than an otl revived Germany, so it will be more focused on strengthening the Mitteleuropa pact.
The Dewey doctrine will stay the point I made.
Nice picture. I think the original was that of a Wehrmacht soldier shaking hands with a Nationalist Chinese soldier.
I mentioned before that this version of the Cold War in TL-191 would be a lot more complex than in real life. At the very least, there would be a tri-polar competition between USA, Germany, and Japan at some point in the future while each nation deals with its own internal problems. USA and Germany, more than likely, wouldn't be hostile but very competitive for international influence. It's probably against Japan that the USA would behave like a "frenemy" and Germany may exploit that.
All three nations would use each other under a pretense of convenient friendship for their agendas.