From Forrest Landry’s essay:
Did you know that if we Earthlings did set up a colony on Mars, that the two cultures would eventually diverge over time simply because of communication constraints?
This means that it is also the case that the technological development paths of the two planets will eventually diverge significantly too.
This divergence would eventually reach a point where each planet, cannot ever actually truly track which types of weapons of mass destruction that 'the other world' could potentially have developed and use against their own world.
They each would not know about the other, what specific forms of the total harm that the other could likely cause. They could only know that each had the full capability, at any moment, without any warning, and with no possible defence, to completely and utterly destroy their own world (total ecocide).
This inevitably leads to an unstable 'assured destruction' situation. One or the other of those planets will for sure take the '1st strike advantage' and thus totally annihilate the other.
You don't need a fleet of spaceships. One spaceship could throw thousands of nukes from distance (like meteorites) so that they all reach different parts of the planet at the same time. Add thousands of decoys; the defenders will not be able to shoot down all of that.
If the attack is against Earth, you could instead throw thousands of capsules containing highly contagious deadly airborne virus, releasing the virus in the atmosphere.
Or both. Throw the virus first, the nukes two days later. People will bring the deadly virus to nuclear shelters.