Wortenia Senki

This is definitely one for the war buffs.
The setting is on the grittier and hard-edged side, MC isn’t “overpowered” in the regular isekai sense, but he has a very well-developed battle sense both with regards to actual hand-to-hand combat (a result of punishing martial training since childhood by his grandfather) and what can only be described as precocious leadership skills and tactical cunning for someone of his age and background.

The politics and diplomacy feel largely realistic, though I’m admittedly no expert on these kinds of things. Otherworld summoning is in fact a crucial plot element in ways best left unspoilt, but suffice to say it’s a fairly dark and sinister affair involving a continent-wide conspiracy whose details are yet to be revealed as of this writing.
But the real heart and soul of this manga, and where this work absolutely shines, is in its depiction of battle tactics – it’s properly immersive in the way it puts you in the hot seat of the battle.

If there’s one thing that breaks your immersion/suspension of disbelief, it’s in the depiction of crucial female military figures as cute young girls barely out of their teens (I’m looking at you Meltina and especially Leona, who’s almost certainly a thirty- or forty-something) when in fact the writing would suggest them to be hard-bitten veterans.
All in all, I’d personally rate it a solid 8/10