When starting a campaign playthrough, the first decision you must make is what difficulty to play on.
Recruit
Recruit difficulty is for players who don’t necessarily want to be taxed by vicious enemies, and is the easiest way to get through the campaign. Despite it being the easiest difficulty, don’t expect to breeze through the game. While you can take plenty of damage before dying, you still have to be very cautious not to sprint through the levels too quickly. Enemies in melee range are deadly and kill you quickly, even on Recruit.
Regular
Regular is the default difficulty. It is recommended for most players. Regular difficulty will offer regular challenges to players new to FPS games. If you are an experienced FPS player, stick to Hardened or Veteran. If this is your first FPS, Recruit is probably a better choice.
Hardened
Hardened will give a challenge to experienced players, but won’t be as frustratingly difficult as Veteran can be. There are several new types of combat in Infinite Warfare, and this may be a good difficulty to learn the ropes of the campaign before trying for the full Veteran challenge. Note that, unlike previous Call of Duty games, there are no special Achievements for beating levels on Hardened difficulty, only Veteran.
Veteran
Veteran difficulty is the ultimate challenge, and recommended for experienced Call of Duty fans. If you remember fondly playing Modern Warfare, then you’re experienced enough to give this difficulty a try. Veteran isn’t as insanely difficult as some of the series entries, but the space combat sequences will require extensive strategy and patience to survive.
The campaign walkthrough is written on the Veteran difficulty level.