"The Will Of The Single Man" versus "No one fights alone"
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 MacMillan 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 MW3.
The Four Horsemen of the Apocalypse and the Ultranationalists
For those unaware, The Four Horsemen of the Apocalypse is a Biblical story that, upon Judgement, God would release four horsemen upon the world, Conquest, War, Famine, and Death. This is paralleled in the series (Most prominently in Modern Warfare's Achievements by name of 'The first-fourth Horseman', obtained by beating certain missions on Veteran difficulty, and some files in Modern Warfare 2. It's pretty laid out who the horsemen are, Makarov, Al-Asad, Zakhaev's son, and Zakhaev himself. But how do they line up?
The First Horseman- Conquest. Makarov
Makarov is the first, Conquest, as by the end of the trilogy, he holds the Russian military under his thumb, and had the plans succeeded, conquered the world with the Ultranationalists. He gave the order to detonate the nuke in the Middle East, was Zakhaev's right-hand man for a long time, and became the Big Bad after Zakhaev bit it in Modern Warfare. Makarov fits the bill for the First Horseman (Which Modern Warfare tries to make you think it was Al-Asad, more on that in a minute.)
The Second Horseman- War. Al-Asad
Al-Asad is the second, War, due to his initial coup,detonating the nuke, and collaborating with the Ultranationalists for seemingly the downfall of America. Nearly everything he did was practically spitting in the face of the U.S., a tactic seemingly out of a different horseman's playbook, but at the end of the day, Al-Asad is the WarHorseman.
The Third Horseman- Famine. Zakhaev's Son
The third horseman slot goes to Victor Zakhaev, who commanded the Ultranationalists in the field before committing suicide to evade capture by the SAS and USMC. Famine is usually associated with loss of necessary resources, such as manpower, food, and weapons, which Zakhaev's son clearly helped enforce on an Ultranationalist-led Russia. Perhaps Russia could recover, without...
''The Fourth Horseman- Death. Zakhaev"
The final (and worst) of the horsemen, Death was supposedly the final warning. Zakhaev pulls this effortlessly, what with the warhead launch in Ultimatum and All In, the consequences for the enemy forces not obeying his warning to leave. Zakhaev is the cause of the entire trilogy, with his wounding by Price cementing his hatred for the West, his later actions causing the deaths of millions, and his party after his death would go on to practically cause World War 3. Perfect for the role of the fourth horseman, who made everyone fear him.