Notes: 

Military costumes are super easy - if you are a General - wear the actual costume or solid colors to match a General's hat.  

Sailors, and boat captains (who many used to be military), can just wear a sailor or captain's hat or go all out with the full sailor/captain costume. 

Paratroopers - you don't have to drag a parachute to the party, but at least wear a flight suit or camouflage - that's good enough! Navy Seals can get the kit with the vest that says 'Navy Seals' and wear all black underneath. 

It's always great for combat characters to smudge a bit of camo makeup on your face for a fun effect. 

You can also opt for an easy T-shirt to reflect which branch of the military your character is in. 

Have fun!