Western Military History.
Compared to military development oy my own isolated nation of japan, or nearby china that most people in my country take interest on. The Europe faced more changes from their own tactical sophistication or external influences like nomadic people coming in and out.
It's practically impossible to learn everything about military history even limited in this region.
I don't know if it's weird, I think the fascination with warfare is very common with people like me. But rather than digressing in surface imagery of roman legion with stereotypical armor and "shield wall" tactics, I decided to learn what they actually were.
Once I get past a surface level knowledge of wikipedia and strategy games, there are so much to learn.
Some 15 minutes youtube video about roman legion will not teach me anything, I'd like to read a book about it or listening to lecture fromyoutube channel.