A series of facts were combined.
1. Big rocks in space are hard to see. We are still finding things bigger than Pluto.
2. There are a lot of rocks in our solar system. Over a quadrillion of them. Hsiang-Kuang Chang estimates a quadrillion small bodies of 100 metre size is probably accurate within an order of magnitude.
3. Big rocks that are moving at high speed are very destructive. This link found by Philip Huggan helps to calculate the destruction and effect of different size objects at different speeds
4. Asteroid deflection is a hard problem even when it happens by chance and we have detected the problem years before it is going to hit. Wikipedia discusses near earth asteroids Asteroid deflection strategies are discussed at Wikipedia The European space agency discusses and analyzes the problem.
5. An attacker can search through the quadrillion possible rocks and find one big enough to do the desired damage, but not too big for them to move and one that is not that far from the trajectory that they want. They can take years to slowly deflect the rock from a near miss to a direct hit. They can also make the rock tougher to spot when it is farther out and before it is brought in closer.
The attacker could adjust the albedo by altering the surface.
An attacker advanced enough to move fairly freely in space would also probably have pretty good mastery of metamaterials and could make the rock invisible and many wavelengths
One might ask: but nuclear weapons are devastating. Why would you need big space rocks ? Nuclear weapons have mechanisms which can be destroyed prior to the time that they get within a few kilometers of their target. Space rocks are destructive because of kinetic energy. Even if you blast the rock to pieces, all of the pieces still retain the energy. If you can blast the electronics of the nuclear device it could be stopped from exploding. Plus as seen in the calculator space rocks can already by very big and fast and big enough to be trillions of times as destructive. A big space rock could create a thousand mile crater and melt the top several miles of the planet.
So if you have advanced space travel, space construction capabilities and metamaterials: you can go to big space rocks, set up camp, establish the means of steering them, use metamaterials and make an invisible shell. Maneuver them into suitable station keeping -ready to use orbits. Maintain communications and protocols with the side that you work for. Then your side has invisible doomsday devices. They are usable for a very messy first strike. Very good for second (retribution) strikes for assured destruction of enemies. The second strike would be good for rational powers to use to deter rational opponents. All rational people would just need to agree to get rid of/completely neutralize those who are dangerously irrational and/or nihilistic.