To unlock the War Bonds trophy in Battlefield 1, you will need to earn 450 War Bonds in multiplayer.
This trophy will come naturally as you progress in multiplayer before you reach rank 10 and unlock the Corporal trophy below:
You earn War Bonds as you rank up in the game's various multiplayer classes. You can then spend your accrued War Bonds to unlock new items. The cost of new unlocks can be found below:
Primary Weapons - 200 War Bonds
Side Arms - 150 War Bonds
Pilot, Tanker and Cavalry primary and side arms - 150 War Bonds