Haha! When I came up with the channel name I was playing 40K and Star Wars Legion more regularly - barely touched them in years so the “Aliens” part is severely lacking!
There's Harry Turtledoves World War series - The aliens arrive in WWII thinking they'd be facing mounted knights. If they'd left a lot earlier, they might have. and Poul Anderson's The High Crusade, where physically weak but technologically superior aliens land in Medieval England. Things do not go well for them