Military symbols provide an alternative to the profile icons used in TripleA,
Contemporary NATO symbols can be found in File:App-6aHandbook.pdf.
For late cold war, see APP-6.
The symbols used during the Korean War can be found here [1].
The navy does things differently in MIL-STD-2525. Pages 7-9 are most useful.
Abbreviations for naval vessels can be found in the wikipedia article [2]. The latest naval symbology can be read in File:FILE N 1000508.pdf.
If you are using military symbols I would suggest putting the nation's flag immediately below the unit symbol at the same size.
These colorful symbols are from Fort Sill's webpage.
These symbols are contemporary symbols from HistoryOfWar.org. Good treatment of air & naval forces.
A somewhat different art style:
From The Armchair General
From Decision Games
Some individual symbols
Anti-aircraft Artillery
Armor
Air
Airborne Artillery
Airborne Infantry
Artillery
Cavalry
Chemical Warfare
Chemical Artillery
Engineers
Infantry
Medical
Ordnance Supply
Quartermaster
Signals
Tank Destroyer
Transport
Veterinary
There is a good page at Wikimedia