Do it Hitler style. He did something that would generally be impossible under normal circumstances- In your world/book you could create some sort of conflict that requires immediate attention.
Hitler became a dictator not by means of military, but by means of his people, Hitler became a dictator because his people wanted to give him the power. The great depression was an immense problem for the people of the 1900s, and Hitler told them that he could solve all of their problems. He was smart, not stupid, the aliens should be able to take his example and conquer the medieval lands. Rule with an iron fist cloaked in a velvet glove, or something like that- Ceasar.
You'll want to make the aliens seem like they have good intentions, and they very well may have good intentions, you never disclosed that information. People in the middle ages weren't exactly as 'informed' as we are today, and they would likely be willing to believe pretty much anything.