There is a great short story with the unfortunate name "The man who came early". It's about a American GI who is stationed at Keflavik during the cold war and s

There is a great short story with the unfortunate name "The man who came early". It's about a American GI who is stationed at Keflavik during the cold war and somehow gets thrown back in time to the Icelandic saga age. He wants to give people there so kind of technology that's much simpler than a smart phone and it all fails. Even something simple as gun powder doesn't get off the ground. They don't know how to get Salpeter. Even if the had the powder they would be missing so much technology to make the guns. As the main character says "you haven't the tools to make the tools to make the tools". Also resources.