The mixed-up influences heard in "London Calling" create a peculiar atmosphere in the song. "London Calling" enters the arena of the vital, fun, punk anthem at the same time that it nears the zone of apocalypse. This song is on edge, yet also a huge party at the same time.
There's a lot to be said about the fun of revolution. "Punk" might have been dead for two years by the time that "London Calling" was cut, but The Clash continued to play on punk's central themes of anti-establishmentarianism and anarchy, though in an intelligent and often political way. The sense of revolution is not just lyrical, "now that war is declared and battle come down," but in the marching beat of the drum. The beat is sparse, alternating between simple quarter note hits on the snare and a swinging backbeat progression. The quarter note hits on the snare evoke military drumming (as does the heavy use of the snare in general; it is a military instrument) and revolution. But the song is quick to transform this into its rock n' roll beat and switch freely back and forth, as if to emphasize that the fun of war is what is coming through in the evocation of military drumming.
Just as with the lyrics, the fun and spirit of the march seems to act as a counterpoint to the darker spirit of the song. Just as Strummer's line "London calling to the zombies of death / Quit holding out and draw another breath" releases some of the dramatic tension of "London is drowning and I live by the river," the marching reggae contours of the song keep the song from remaining totally dark, with its wrenching guitar solos, howls, and unresolved ending complementing the apocalyptic lyrics.
Take the intro, for example. The song begins with its E minor/F major chord progression, but after awhile Mick Jones' overdriven lead guitar stops alternating, hanging on that funky Fmaj9 while Strummer alternates between the E minor and F major, increasing its bite as the tension of the unresolved progression grows. It's wicked.
Then there's the bridge section after the second chorus. The hum of feedback makes way for the howling of Joe Strummer at 1:55 and the echoing guitar solo, which sounds like the twisting of metal in the punk revolution that could have been.
While the anarchy of the sound is fun, it's also frightening. Strummer's howling sounds like that of a rooster, perhaps symbolizing not only the rising of a new sun, a new era, but the wild anarchy of that new era. But it also sounds like the crow of rooks, omens of death. Or could it be the howl of a wolf?
Then there's the end of the song. Typically shorter pieces of music like pop songs go in a sort of musical circle, beginning a certain way, on a certain chord, and ending with a return to that chord, called the tonic. This song doesn't resolve at all. In fact, the end is made out to be distressing. Instead of ending on the tonic, the song echoes out with Strummer's last line, "I never felt so much a-like..." never completing. If the mission of punk is to rebel and provide a pulse to anti-establishment sentiments through its lyrics and sound, then perhaps the worst fate of punk is to not get that message out. Ironically, the only message that does come out is SOS. Jones switches between his pickups to spell out Morse code with his feedback (it is common for guitars to have noisier pickups that buzz when the guitar is set to read them). If many were looking to The Clash as the next important British band after The Beatles - and they were; many billed them as "The Only Band That Matters" - then The Clash is here communicating their own inability to have the proper voice, as well as their own sense of being lost.