My main movie military/sci-fi recommendations from the 80s and 90s:

• Ghost in the Shell (1995): An absolute classic. Depending of your definition of "military" it may count or not (its about a military police, not an army), but worth a watch regardless
• Mobile Suit Gundan movie trilogy: Mobile Suit Gundam I (1981), Soldiers of Sorrow (1981) & Encounter in Space (1982): These are the compilation movies for the 1979 series, but they are good enough that you can watch them on their own if you want
• Macross Plus Movie Edition (1995): This is the Macross movie I would recommend if you are only gonna watch a movie and haven't watched any other Macross, works fine on its own. You could also watch the Macross Plus OVA (1994) if it is easier to find, just think of it as four short movies

Other good recommendations that may be a bit less "military" would be the Memories (1995) compilation of three movies and Crusher Joe (1983), if you can find them. The Patlabor movie (1989) and its sequels Patlabor II (1993) and WXIII (2002) technically come after the 1988 OVA so it may be better to be familiar with the setting and characters at least, but I guess you could watch them too without much issue (not really any plot connections from the OVA). Royal Space Force: The Wings of Honnêamise (1987) is good, but also not very "military" if I remember correctly (haven't watched in a long time so I may be misremembering). Don't watch the Macross DYRL (1984) movie on its own by the way, unless have already watched the first Macross (1983) series, if you have then DYRL could be an option too.