Call of Duty: Modern Warfare/Analysis

"The Will Of The Single Man" versus The Power of Friendship
One of the big selling points of the original Call of Duty, if those of you old enough to have played that game will recall, was that you were no longer expected to win the war single-handed, that you would have allies on your side (though the usefulness of your allies might be debateable). In the intro monologue for Modern Warfare 3, Makarov states that all it takes is the will of a single man to create a global conflict, scoffing at the world leaders' attempts to shape events around the world in their favor.

In fact, if you look at the villains of the series, you get a distinct impression of We ARE Struggling Together bordering on With Friends Like These. At least one of the villains was actively working against his biggest ally, causing an Enemy Mine situation between the other villain and the heroes (of course, that first alliance was an Enemy Mine situation to begin with...) It is mentioned in the second game that one of the things that Zakhaev had been up to before his death was keeping his underling Makarov in line. Makarov's own actions end up directly driving one of his own companions into the enemy camp. None of the bad guys seemed able to actually work together towards a common goal.

Compare to the Good Guys(TM). The Brits and Americans pass intel and assets back and forth. The Americans give the SAS rides in and out of various situations, the SAS and Marines form joint operations to go after the Big Bad in the first game, the spec ops forces of various allied nations get together in the second game to form a joint unit. The Rangers of Hunter 2-1 only survive because they are able to work together and stick it out no matter how bad the situation gets. In the third game, Sandman and Mac Millan are willing to give Price the benefit of the doubt, despite him being a wanted fugitive, based entirely on having fought alongside him in the past.

It would appear that a central theme of the Modern Warfare series is that much more can be gained from working together than from working alone. The villains were constantly having to keep each other in check, waiting for the inevitable knife in the back, while the heroes were willing to work together even when they may have had reason to distrust each other. Even when Price echoed Makarov's own line about the Will of the Single Man, he had the help of his True Companions, which was instrumental in his success. Thus, the heroes ultimately prevailed.
 * To add to your statement, there is also the intro of the first Modern Warfare: When Al-Asad meets Imran Zakhaev in the execution square, he points his desert eagle at him before handing it over, as if to inform him who's actually in charge. In any case, they certainly don't trust each other. In contrast, has there really been any time during the campaign where you don't wade into hostile fire to save your friend's lives? Jackson and the marines went back to save Pelayo. Nikolai went to Afghanistan to save Soap and Price. The Czech Resistance comes to your rescue in MW 3.