Basically, if you've already played the game, you should know that there are several endings to it. If you're too lazy to look up the steps and processes, then I will list them here. Please note, that this guide will have spoilers, so make sure you complete the game before reading this.
Pyrrhic Victory: Rescue Woods Celerium: Recover "Worm" Old Wounds: Interrogate Kravchenko (Resist Shooting Him) Time and Fate: Find "CIA Memos" Fallen Angel: Gather Intel (3 Parts) and Protect Harper Karma: Kill DeFalco and Rescue Karma Suffer with Me: Shoot Mason in the Legs (Twice) Achilles' Veil: Kill Harper. Farid Survives Odysseus: Save Briggs and Defend the Ship Cordis Die: Protect the President Judgement Day: Capture Menendez
Strike Force Missions
Shipwreck: Freighter Destroyed I.E.D: Afghan President Saved FOB Spectre: India Protected Dispatch: Assassinate Tian Zhao
Miscellaneous Strike Force Mission
If DeFalco escapes in "Karma," this mission will trigger after the initial story mission. This gives you a chance to rescue Karma.
Second Chance: Rescue Karma
Menendez is captured by David and escorted away (Brief Dialogue). Frank Woods is confronted by Alex Mason in his home ("The Vault"). Karma, in a cutscene, is the only one able to defeat the Celerium Virus. David goes to see Frank, and enters the room to see Frank and Mason, who he does not recognize as his father. Frank introduces them as "Al," and "Dave," then tells David that "Al," is his father (Dialogue). Flashbacks occur, more dialogue. Karma is seen on an interview that Menendez happens to be watching. Menendez then proceeds to hit his head against the television in frustration. Game then ends.