I'm trying to ID a story I read in the local library the early 70's when first getting into SF. I'm pretty sure it was a short story in a collection, but can't rule out it being part of a longer novel.
It was about a man in a room/lab, hooked into some sort of remote viewer, through which he could use telekinesis or similar to take part in a war with aliens in a distant solar system. I think the machine augmented his powers in some way, as he was able to bring ever-more ferocious attacks to bear, to the point where he completely melted the surface of the alien home planet, turning it to swirling molten rock. Getting carried away, he was about to destroy all the stars he could see from the viewpoint of the alien planet to make sure there was no chance of any aliens surviving, when he realized that one of those stars might be Earth's sun, and stopped himself just in time.
I can't give any more details, that's all I remember.