How to write a war?

Not a battle, no, but a war. How can you portray a war in writing format? From the strategy and formations to the consequences of the environment on the soldiers, the geostrategic and geopolitical aspects of the war, the logistics, and so forth.

You're looking for the Combat-Writing Thread I think...

Basically? You just have to spend a LOT of time on Deconstruction (if you're pessimistic), Reconstruction (optimistic), or both. Just look at Game Of Thrones and how long that's been going on.

There are just too many variables to answer that quickly. The short answer is "read history and draw parallels".

For longer answers, come on over to Combat-Writing.

To write a story about a war, first decide whose perspective you're going to tell the story from. Is it the story of a lowly infantryman, who would naturally see the war from ground-level, and be more focused on the intimate details of tactics and combat? Or is it the story of a high-ranking general, who would get a much broader overview of the war, and the strategy and politics behind the operations?

All stories need to be about characters. This is true even for high-concept war stories. A story cannot simply be about an idea - there has to be a human element for people to connect with, and give the plot some context.

How much do you remember of the War On Terror? How much of that time could you learn from other people who were differently-positioned than you in their life, during that time? From which angle would you want to retell it?

For some people, it's just watching the news a lot or conspiracy theories but otherwise nothing changes in their lives. For others, people they know get drafted and come back dead or maimed, everything is rationed, and/or there's a job vacuum. For other people, it's everybody else suddenly becoming a lot more racist than they previously were. For other people still, it is actual bombs and bullets that they have to dodge either as civilians or soldiers. Other times, it's martial law and curfews and no longer any privacy.

It depends on the war, and who is where in the war, and how connected or disconnected you make everything.

It depends on the story. Most of the other details could be written as inferred, rather than heart-of-no-mans-land battle...or both.

It depends on your timeframe. A medeval war with swords, arrows, etc. will be very different from a war set around the American Revolution, and both will be completely unlike a modern war.