I disagree. I believe the problem is with the relevance of the "one person" you're playing - and the player base.
Telling a war story doesn't mean you have to be the only relevant person in that story.
The bigger trouble is the general expectation of RPG power fantasy. People want to feel like they determine the course of the war, rather than role-play with the war as context to be reacted to.
Personally, I think an excellent RPG context would be if the PC was on the losing side of a war. Where even if the PC wins the battle here or there, the battles beside them force retreats, odds get impossible as retreat really is the only option, and towards the end the enemy gets far more OP than the player. Come the end, the war is lost, the PC leads the last holdouts, and the final choice? To surrender, or refuse to give up and fight to the last.
If I had to pick an example, Japan in WW2 would be a great example. The war strategy ultimately depended on the US losing the will to fight. Barring that, the industrial miss-match was so large that even victories would be delaying the inevitable and heightening the final cost.
But just think of the roleplaying opportunities for the player. Pulling off a major victory that doesn't actually make the enemy give up? Do you choose to try and sue for peace early, before it's too late? (Hardliners- who would be your allies if you didn't- oppose you.) Do you resort to desperate measures in the name of defense? Do you accept increasingly high costs at home to maintain your war effort? And how do you react when the enemy breaks out the nukes?
(Though you got to admit- it'd be kind of cool if the 'last stand' option could end with a 'victory' where the enemy stops trying to overrun you and just drops a nuke on you. Great epilogue slide potential there.)