Questions like this can be easily researched online! You should read about the time period you are writing about before you launch into the story. Nothing is more frustrating than writing an entire story, then doing research, only to find out everything you wrote has to be changed.
As a side note, there is a such thing as historical fiction. You could write a story on the premise that “this is what would of happened if France and Germany were allies in the 1400s.” You can also do something like “France and Germany had an alliance, and did this, but because they were defeated by (insert rival country here) in the battle in my story, all records of the alliance and this war were intentionally left out of the history books. After all, the victors get to write them.”
