In recent conversations with a friend, we both realized that there was this gab in our knowledge of European history. Because the relationship went from basically several hundreds of years of mortal enemies, to “suddenly” (by our knowledge) being allies and fight the great war together. What happened in between to change this relationship?
Here's a few older answers from the FAQ: https://www.reddit.com/r/AskHistorians/comments/2c1p69/comment/cjbbfha
https://www.reddit.com/r/AskHistorians/comments/2cpyy7/comment/cjiazrp
Both by u/elos_ if the FAQ is to be believed, although they've deleted their account.
TLDR: in the years running up to 1914 both countries found common cause in feeling threatened by Germany.