Is this an issue for older military designs, like, a Sherman Tank, or Maus, or even older like a WWI plane like the Fokker?
For copyright law anything after 1922 is out of bounds, don't use it.
Trademarks are restricted as long as the company is using the name, with some trademarks dating back to the time trademarks were first introduced in law, and some have been in continuous use for hundreds of years.
As for those specific items, a few moments of research show it is absolutely an issue for both the Sherman Tank and Panzer Maus. For the Fokker planes it is only probably an issue.
The Sherman Tank is still in active use; expect to be taken down if you use it without permission. The older Panzer Maus tanks are not in active use but Krupp is now ThyssenKrupp which still apparently owns everything involved and makes money from licensing; again, expect to be taken down if you use it without permission. Fokker declared bankruptcy and sold their assets in 1996 but has since been reconstituted from sold-off parts. The current incarnation of the company probably still own the old IP rights. If you use their airplane designs you might be fine, but if you use the Fokker name expect to be taken down if you use the name without permission.
If you are making a game where you need historical accuracy you need permission and there is no way around that. A few years ago EA tried to test that theory by not licensing a few names, was taken to court, and ended up changing tactics and acquiring licenses anyway. I'm sure your legal fund is a lot smaller than EA's legal fund.
Make your game your own. Build your own style of box with a turret for a tank, your own style of box with wings for airplanes. Build your own game.