It's easy to overstate the threat proposed by a potential enemy - just project your worst fears on them. I saw a lot of that during the Cold War. The Soviets were ten feet tall. They had millions of tanks and planes. We didn't have a chance.

Without going into any details (because I don't know the classification of this info), I've heard plenty of concerns similar to the ones expressed here about potential space threats. The key question to ask is, "Can we do this?" If the answer is no, then odds are the other side can't do it either.

There are serious design issues with making nanosatellites that can accomplish such missions. For one thing, the smaller the satellite, the less volume is available for propellant. While less mass means you need less propellant for a given delta-v, there comes a point when a satellite is so small that you simply can't put enough propellant into it to accomplish much. Renvedous and docking (or attaching) with another satellite requires a fair amount of maneuvering, which means you need propellant. Remember, the volume of a sphere increases with the cube of its radius. Doubling the radius increases the volume by a factor of 8. Unfortunately, the converse is true. Reducing the radius by half cuts the volume by a factor of 8.

volume of a sphere = 4/3 * PI * radius^3

Radius/Volume
5 cm/523.6 cm^3
10 cm/4188.8 cm^3