I am trying really hard to remember where I saw this. I remember it took place in a modern world, but there was a field or dome that they could see in to. The beings inside looked 1800s or amish and appeared to be human. At one point some military type guys go inside and turns out they weren't human at all and are attacked by them. Thats all I really remember, and I can't find anything anywhere.
This sounds like the Night Visons segment A View Through the Window, based on the short story Window by Bob Leman. The army investigates an idyllic ca 1800's prairie homestead which been found in the middle of the desert.