1948: Space Cadet, a novel by Robert A. Heinlein.
From the Wikipedia summary:
The Space Patrol is entrusted by the worldwide Earth government with a monopoly on nuclear weapons, and is expected to maintain a credible threat to drop them on Earth from orbit as a deterrent against breaking the peace. Matt, on a visit home, causes a family argument when his parents refuse to believe that the Patrol—and especially their son—would actually bomb Iowa.
Excerpt (transcribed from the 1978 Ballantine Books edition, fourth printing, October 1985):
One night at dinner his father had asked him to describe just what it was that the Nobel did in circum-Terra patrol. He had tried to oblige. "After we lift off from Moon Base we head for Terra on an elliptical orbit. As we approach the Earth we brake gradually and throw her into a tight circular orbit from pole to pole—"
"Why pole to pole? Why not around the equator?"
"Because, you see, the atom-bomb rockets are in pole-to-pole orbits. That's the only way they can cover the whole globe. If they were circling around the equator—"
"I understand that," his father had interrupted, "but your purpose, as I understand it, is to inspect the bomb rockets. If you—your ship—circled around the equator, you could just wait for the bomb rockets to come past."
"You may understand it," his mother had said to his father, but I don't."
Matt looked from one to the other, wondering which one to answer—and how. "One at a time . . . please," he protested. "Dad, we can't just intercept the bombs; we have to sneak up on them, match orbits until you are right alongside it and making exactly the same course and speed. Then you bring the bomb inside the ship and inspect it."
"And of what does that inspection consist?"
[. . . .]
"Clear enough. And these corrections have to be made often enough that a ship is kept busy just inspecting them?"
"Well, no, Dad, we inspect oftener than we really have to—but it keeps the ship and the crew busy. Keeps it from getting monotonous. Anyhow, frequent inspections keep you on the safe side."
"Sounds like a waste of taxpayers' money to inspect too often."
"But you don't understand—we're not there to inspect; we're there to patrol. The inspection ship is the ship that would deliver an attack in case anybody started acting up. We have to stay on patrol until the next ship relieves us, so we might as well inspect. Granted that you can bomb a city from Moon Base, you can do a better, more accurate job, with less chance of hitting the wrong people, from close by."
