Agile. You do a waterfall based plan at the beginning (the very formalized operations order) that cascades from the highest level all the way down to the squad level. Operations start and then you iterate.

Most important two sentences in the operations order is the mission and commanders intent: If all else fails, get this done. And this is why.

This leaves lots of room (and expectation) for iteration and innovation along the way as things inevitably get f'd up with a dynamic enemy.

Periodically, you will refresh the opord and start the iteration again. This is practiced ALL the time by combat units at every level through simulations and on the ground training.

Source: was an infantry officer.

One of the adages of warrior class is that plans never survive contact with the enemy.

Think more like a function, continuously iterated. While War = 1 Observe; Orient; Decide; Act;

https://en.wikipedia.org/wiki/OODA_loop

Closer to Agile, Waterfall implies a commitment to your early plans despite the reality of the situation once the action starts. Real world military projects are iterated on frequently and adapted once they hit the field (and reality).

I asked a similar question a few weeks ago [0].

The Pentagon Wars [1] is an interesting book about how military systems are developed. They also made a movie.

0: https://news.ycombinator.com/item?id=32056507

1: https://en.wikipedia.org/wiki/The_Pentagon_Wars

For the US Army, you can read through ADRP 5-0 which outlines the Operations Process.