A Hitch in Time — Frederik Pohl

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. The sentry challenged me, of course, but I was ready for him. After a quick look at my stolen credentials he sheathed his ray pistol.