Alright, first you're gonna wanna go for certain outfit types of apparel, but the main two you will want to use are
Army Fatigues

and Military Fatigues

But if you wanna wear something else then try and justify it being during that time period (1914-1945)
Also I'm allowing ballistic weave because that was an actual thing during one of the World Wars, but I forgot which one though...

As for armor, you will not be wanting to use these armors...
Synth Armor
Combat Armor
Assault/CoA Marine Armor
Pack Raider Armor
DC Guard Armor
Robot Armor
Trapper Armor
Vault-Tec Security Armor

But armors you CAN use are...
Metal Armor
Leather Armor
Raider Armor
Disciple Armor
Operator Armor

As for helmets, you can only use these helmets/headwear
Army Helmet
Metal Helmet
Gas Mask With Goggles
Green Hood
Bowler Hat
Fedoras
Minuteman Hat
Oswald's Tophat
Metal Helmet
Military Cap
Newsboys Cap
Press Cap
Green Rag Hat

And lastly, NO POWER ARMOR, they didn't have that stuff in WW1 and WW2