Grave of the Fireflies is a classic.

The Saga of Tanya the Evil does a surprisingly good job of it, considering it's an isekai about flying mages.

Much of it is not-Germany fighting not-Russia while not-Italy are acting as neutral peace facilitators while not-UK are harrying the western flank, and they're worried about the not-US getting involved and screwing up the balance of power. Lots of politics and strategy mixed in with some good action.

I’ll argue the first Gundam

Does Grave of the Fireflies count? It isn't necessarily a war anime, but it does very much depict war and the reality of what it is like to the main characters despite not being in the war themselves.

And just like real war, it is absolutely fucked up.

A Witch in Mongolia

I've heard good things about Vineland Saga not shying away from its depiction of generational war. Somebody told me it's one of those where everyone kind of sucks but it will give you context to why the violence is happening. Been on my watchlist but I haven't gotten to it yet (I'm dragging my feet because I've come to hate vikings in media)

Flag. It's a real robot mecha set in modern times, the mecha is basically a tank. And its told as a documentary.

I think "In This Corner of the World" does a pretty good job.

Now and Then, Here and There

It’s pretty unflinching but not nihilistic.

Pumpkin Scissors.

War is easy. The aftermath is hard.

Izetta and GOSICK come to mind

Really depends on the era.

The Cockpit might be of interest (if you can find it—I'm not sure there's any current streaming license). Zipang, too. There are probably others I'm forgetting about.

Gate is pretty realistic for the whole modern warfare vs ancient roman warfare.

edit: to be clear, the show was very creative on the effectiveness of the JSDF. watching the show would make one think they are some kind of supersoldiers. however, the tactics and the equipment used is consistent with modern military warfare. the only thing missing was drones, though that can be forgiven due to the age of the show and the source material.