How do I drag most of humanity into an apocalyptic war?

Question

In the future world (based in late 22th century, main place will be Earth) I am creating, I need a giant war so I can make the post-apocalyptic story happen in the first place. The issue is that it is hard to do that in the first place, given how spread humanity has became.

Current human capacities

Humanity has spread to the asteroid belt, space (Lagrange points and geosynchronous for human colonies). Moon, Mars, and Jovian and Saturnian moons. The Moon is a country unto itself, Mars has 10 countries, the asteroid belt is reserved almost exclusively for mining, the Jovian and Saturian moons are base colonies, and space is experimental purposes and hotels. Pretty much everyone has nuclear capacities, and greater (the ultimate weapon that only a few countries have is antimatter nukes, due to UN treaties, so they must be secret). The countries have only slighty reduced, due to increased fragmetation before antimatter could be used as energy. Most military use non-lethal lasers. Assemblers have been made, but you still need to get stuff

World situation

The world (Earth) is still tense from a recent cold war, and global warming. The big country (22th century equivalent of US right now) is Brazil. (China/India had some problems, long story). The main languages are Portuguese, Chinese and Untrea (popular international language). The main currency is Bitcoin. Aliens have been discovered, but only bacteria. However, mysterious signals have been found by future SETI.

Rules

1. I am OK with trans-asteroid belt moons being spared (only like 105,000 anyways)
2. I would prefer on Earth, most likely will be most approved. Implausiblity OK, just not "Harry Potter" level (or AI). (i.e., "Misfunctioning translator" over "ALIENS FOUND!!")
3. I will add any details asked for in the comments.

Answer 1

A religious offshoot started by a wing nut living in the asteroid belt thinks those on Earth have it easy and they are destroying humanities birthplace. So he sends an small asteroid, the size of a semi-trailer toward Earth, He puts an ion drive on it to speed it up. This also makes it look like a space ship which he uses as a disguise to get close up. When it hits Earth it causes huge environmental issues killing billions and sparks an interplanetary war. Since the nut job was on the asteroid, no one was left to take credit and everyone is blaming each other for the atrocity. Earth was left to fend for itself for generations while everyone else started strike and counter-striking each other.

Answer 2

Regular thermonuclear war, but be sure to take out the space elevators and spaceports.

The space and moon colonies aren't going to save Earth. You said the space aspects are primarily for mining and "experimental purposes and hotels". The countries on Mars and the Moon would certainly rely heavily on trade with Earth. That means without resupply from Earth, they're going to die out there. If they do make it to, or were in, Earth orbit the space elevators are gone, too bad most ships are designed to dock with them and can't reenter the atmosphere. They'll likely have small escape pods capable of reentry, but they're not designed to help someone survive on the surface for long periods. Reentry is especially problematic if their only return option is a crash landing somewhere outside an irradiated zone.

It may actually help your plot to have a few of them survive a few decades out there. They'll be like fallout vault dwellers returning to the wasteland.

Answer 3

There is a planet that is the only source of an incredibly valuable substance. This can only be used to make a dreadful type of weapon. There are two empires and neither can risk the others getting hold of the stuff.

When anyone tries to mine the substance and transport it away, the other side tries to stop them. This conflict grows over time until the empires are in constant war.

Answer 4

Sending an asteroid towards the Earth is a Rube Goldberg kind of apocalypse, given the asteroid can be seen from a long distance and it will take years or even decades to move into an impact orbit. Nukes are a lot faster.

On the other hand, ordinary spacecraft are deadly weapons in of themselves because they move at such incredible velocities. Just staying in orbit around the Earth requires you to be moving at over 7847 m/s at @ 100km, so impact with anything will be catastrophic. Crashing spacecraft into each other will eventually lead to a cloud of high speed debris in orbit, making approaching or leaving Earth very problematic, at best. Similar conditions would obtain around Mars and the Jovian moons, although the actual velocities will be different (generally much higher, since velocity must be added to the Earth orbital velocity just to get there. Crashing space vehicles don't need to consider burns to shed velocity in this case).

WRT attacking the planets themselves, similar considerations apply. Space junk can become deadly meteors (especially if actually aimed at targets on the ground), and even low density materials will have a great deal of energy. The Tunguska event in 1908 or the recent Chelyabinsk object were essentially snowballs which disintegrated in the upper atmosphere, releasing kilotons of energy, much like a small nuclear weapon. For a planet with a limited atmosphere like Mars, this could actually blow away much of the current atmosphere. For a moon or asteroid with no atmosphere, you are getting nuclear scale impacts, which will have severe consequences for life support machinery and structural integrity.

So from a physics POV, doing everyone in is quite possible. What is not clear from your story backdrop is "why" anyone would want to do this? Conventionally, "war is continuation of politics by other means", and politics is defined in Organizational theory as a means of allocating limited resources. What resources are limited, and why can they not be allocated (especially given the amount of energy available to a spacefaring civilization)?

This sounds more like the sort of "tribal" warfare that existed in pre modern times, and could be powered by ethnic, social or religious (ideological) groups which come to see each other as existential threats. If that is the case, what may trigger the final confrontation is the belief that more members of your tribe can survive because they are hidden away on the asteroids, Mars or the Jovian moons, so the calculus of MAD does not apply like it did for the US and USSR during the Cold War.

Of course in this case, the idea that any of your enemies might be able to survive in deep space would lead to a general war of extermination as high speed interceptors are blasted towards every known space settlement in order to ensure the destruction of the other. Few settlements might be able to escape, but given the very limited resource base (especially manpower and machinery), they may slowly die out or choose to try and return to Earth where it is barely possible to survive without a technological envelope around you.