I think this was a short story from the 1980s where two warring species try to deny spaceflight to each other. A seeded B’s low orbit with essentially ball bearings to deny safe launch capability. But B did something clever with magnetic fields (magic!) to clear it. Then B did the same to A but with sand. Sound familiar? Might have been Analog or similar magazine.
"The Neighbors", a short story by Joseph H. Delaney; published in Analog Science Fiction/Science Fact, January 1986, apparently never reprinted. One of the warring species is us Earthlings, the other is the beings of the planet Thassa 70 light-years away. There is no FTL travel, just generation ships. Two excerpts.
In Washington, DC:
"We can't get up there to find out for certain, but the speculation is that somebody's dumped the equivalent of a couple of tons of buckshot into low-Earth polar orbit. They won't know for sure until we can develop boosters big enough to carry armored capsules.
"But if that's the case, long before we can do that our high orbit sensor and communication satellites are going to poop out—ours AND Russia's. We'll be blind. So will they. We'll both have cause to be nervous then, and I don't have to tell you, anything could happen."
It was the President's turn for a red face. He hadn't thought of that.
"Of course, having that stuff up there, assuming it is, will take some of the risk out. Missiles in fractional orbits will be just as vulnerable as satellites, but—"
"—But we can't rely on that. No, sir, we've got to talk to them—find a way to sweep this stuff up—find out who did it in the first place and keep it from being repeated. I'm commissioning your agency to beef up its surveillance capability enough to sweep the entire solar system and find out who's out there, and I intend to ask our enemies to do the same."
"It's a tall order, Mr. President. And we're dealing with a pretty suspicious bunch. Maybe they'll come around and maybe they won't. If they don't, well, we still have the capability of mutual destruction, even without missiles. They know that as well as we do. Uh—I take it you don't suspect they're behind it?"
A glance at the President's face was enough to answer his question. At once he understood—the Russians were capable of it, and they were rash enough to do it. But they weren't smart enough to do it without getting caught doing it. Suddenly the unthinkable—wasn't.
Later, on the planet Thassa:
"But how, Acadamecian? How could they ever have known?"
"It is impossible for me to say, Your Majesty. These creatures have, nevertheless, destroyed our race. All your might of arms was useless against them, because they came to us in our own ship, and we believed they were our own returning people. It is only a matter of time."
"But why? Why must we sit here and do nothing? Should we not be attempting to sweep it up, as these creatures must have done?"
"It is not the same thing, Your Majesty. We sowed particles of substantial size, with the objective of confining them to their planet. By armoring their spacecraft they could get through it. At enormous expense and effort they could sweep their near-space relatively clean. But this is dust, particles very finely divided and far beyond our ability to remove. And even now, as we discuss it, Thassa cools. In days our mean temperature will fall and our oceans will freeze. It will occur too swiftly for us to do anything. Except for a fortunate few under domes elsewhere in the system Thassalak will be extinct within a generation, and even these, denied access to the mother world, will ultimately perish. Worse—it was all for nothing—we had nothing to fight over. They never were a threat to us, until now."
"No! We were prepared! We were strong! We are still strong! We can still defeat them."