The return of the single-player campaign to the Call of Duty series after the previous game (Call of Duty: Black Ops 4) didn't bother with one is viewed as such by fans. While never the most popular element of the series, a lot of fans have extremely fond memories of the COD campaigns. The fact that it is a Modern Warfare campaign, set in an Ultimate Universe version of the original continuity doubly helps matters.
Garrick proves to be a One-Man Army worthy of other Call of Duty protagonists in the numerous missions that he undertakes on behalf of the American and British military.
He gets double props if he doesn't kill any innocent hostages in the chaotic events of the Piccadilly Circus bombing where there's crowds running in every direction.
Managing to hold off an entire army of Al-Qatala forces trying to take back The Wolf. They fail but he'll save Stacy a second time and make them bleed for it.
He eventually manages to capture the Butcher and can have a What You Are in the Dark moment where he decides to interrogate the Butcher by threatening his wife and kids. Either he does it or doesn't but the Butcher breaks.
Garrick and Price are Bash Brothers that actually has to use stealth tactics, covert assassinations, and things more appropriate for Rainbow Six or RL Special Forces in order to take down the army of terrorists inside Barkov's home.
And then there's Alex, who also proves to be a One-Man Army on his own.
He serves as one of the soldiers who helps bring down an entire Russian military base with nothing more than some poorly armed locals and a few remote control airplanes that can take down numerous heavily armed helicopters.
He can end up sniping a small army of Russian and Al-Qatala soldiers during the "Highway of Death" mission, eventually crawling to safety despite getting a face full of nerve gas.
Alex also chooses to do a Heroic Sacrifice to take out Barkov's chemical weapons factory.
Even that's not enough to put him down, as he returns in Season 3 sporting a prosthetic leg, ready to fight.
Farah goes through a Trauma Conga Line that also makes her an Iron Woobie at the end when she witnesses her father being killed by a Russian soldier and then has to stab him to death multiple times while using stealth to hide in her house. Even then, she only survives by her brother's intervention and stealing a gun to use. From there, she kills two more Russian soldiers and almost escapes.
Farah one-ups herself later in life by escaping from a Russian prison cell, releasing an Amazon Brigade of her fellow prisoners, and pulling off a Big Damn Heroes moment with a bit of help from Lieutenant Price and the SAS. This despite the fact she was suffering ten days in solitary confinement, thirst, hunger, and a bout of Cold-Blooded Torture at General Barkov's hands.
Farah also willingly goes on a mission to capture her own brother despite the fact that she's just recovered from almost being killed by nerve gas.
Farah is the person who kills Barkov in the end and does so with a dozen stabs to the chest on a helicopter she's snuck on. Or she shoots him.
Price has always been a Pragmatic Hero and Good Is Not Nice. He manages to show that in an Establishing Character Moment when he does an Homage to the aborted Rainbow Six Patriots. He proceeds to take a man with a bomb strapped to him and throws him over a balcony in order to prevent him from killing a bunch of other hostages.
Price makes it clear to Colonel Norris that he is not going to let him arrest or kill Farah after her organization is declared a terrorist group.
"Clean House" is a level that shows the SAS at their best in the Call of Duty franchise as they sneak inside a house full of Al-Qatala sympathizers and methodically clean level after level. If the player character is smart, they can avoid being ambushed by a number of female members taking advantage of being The Unfair Sex and Would Not Hit a Girl. They can also prove their Would Not Shoot a Civilian cred by restraining fire with one trying to protect a baby in the location.
The "Embassy" is an enormous and lengthy mission where the American Embassy in Urzikstan is overwhelmed by Al-Qatala forces. You have to fight through their attack, guide a embassy worker through hostile territory, and then seek refuge at a safe house. Then you have to defend that safehouse in The Siege that includes using drones, flares, sniper rifles, and street-to-street combat. You also have numerous Hostage Situation events that take seconds to deal with.
"The Highway of Death" is an excellent sniper mission that involves use of wind and distance to make your shots as you take down Al-Qatala scouts as well as mortar teams. You even get into a brief sniper duel. This is before you have to hold off a small Russian army before Hadir uses nerve gas on them all.
"Going Dark" is a magnificent mission with a combination of stealth and combat gameplay that will make you believe you're in a Splinter Cell game. You must head into Barkov's estate and rescue his family while dealing with an entire army of Al-Qatala assassins, including searching Hadir. Shooting out the lights and staying out of sight is your only way to survive even as you must cross an absolute mammoth map that includes multiple structures: a church, a pool, a mansion, an enormous garden, and a courtyard.