A Hitch in Time by Frederik Pohl

Obviously the man was dying, and there was no chance that he ever would be discovered.

I blessed the carelessness that had caused me to set the space-time dials a little off when I began this journey to the distant past. I had come to this barbaric era in the proper time, indeed, but millions of miles removed from it in space. It had been only after an annoying search that I had discovered Earth, jetted toward it in my space-drive suit and had come down out of the skies to land on this tiny, deserted island in the middle of an empty sea.

But it was incredible luck that had brought me there. For I had found exactly what I needed—a man who would give me information, clothing and an identity—and then die, and obliterate the record of my interference with the course of events!

I, Thom Ra, walked toward him. Feeble though he was, he opened his eyes and stared at me.

"Thank Heaven!" he whispered, in the thick, hideous language of that era. "I couldn't have lasted much longer if you hadn't found me." He fell back and smiled at me with heartfelt gratitude, and for a moment I felt a wild, fleeting impulse to help him, to save his life. But of course, I dared not interfere. For that would change the shape of the future, and that meant destruction for me....

When I blasted off from the island, a little later, he was dead, and I was wearing his uniform—and his name.

He gave me information before he died, and I had no trouble locating the spot I wanted. I waited till dark before landing a few hundred yards from the war-dome. Then I hid my space-drive suit in a cluster of ancient trees, and walked into the building that housed the most murderous weapon of all time.