I find the manga funny.
It's pure military style power fantasy of people that wish they didn't lose WW2...
I tracked down the web novel / light novel (most of which has been translated). Turns out it's not power fantasy of people that wish they didn't lose WW2. It's anti-Imperial Japan, pro-modern pacifist Japan.
The antagonist countries that they blast to hell all do things that Imperial Japan did. If you know the history, you can pick out specific parallels between things the bad guys do, and things Imperial Japan did. One of the antagonists is almost a carbon copy of Imperial Japan (island nation, same population as Imperial Japan, colonial, racial superiority ideology). Author even gives them a ship said to be identical to the Yamato, and names their primary fighter plane the type zero-seven (not type 7, but type 07).
The author is clearly a big fan of the military (repeats specs of military hardware over and over, and over and over). But their political message behind the story seems to be: hegemony bad, peaceful coexistence good, but don't be afraid to use your military power to protect that peaceful coexistence. They're probably expressing their disappointment over Japan refusing to participate in the Gulf War coalition to oust Iraq from Kuwait. (Japan eventually did deploy a unit overseas for the first time during the UN peacekeeping actions in the Balkans conflict.)
I expected the magic stuff to have a chance... but it's sadly just a one sided slaughter.
Towards the end of the Gra Valkan arc, the top magic nation does bring out hardware which performs better than Japan's (after they use their observations of Japan's weapons to guide their R&D). But they're allied with Japan so don't go at each other.
The story is pretty obviously setting up the Ancient Sorcerous Empire as the BBEG. I still have a few volumes to go, but they have yet to show up. Only some experiments they left behind which became the demon lords, which were easily dispatched with modern weaponry.
The story does make a big deal about how a few decades of difference in tech level can result in a huge difference in power. Which is probably the author's pro-JSDF opinions showing through. They want the country to provide enough funding so the JSDF can acquire the latest hardware. And this story is their explanation why.