Greek fire: We have ideas of what it might have been but no idea how they did it or what it was. link to google

English long bow - we can make one but I don't believe we can shoot them like they did back in the day.

Cavalry - We can breed horses but we don't spend the time on them that people used to. I don't think we could put a decent heavy cavalry together quickly.

Sorry, we don't allow throughout history questions. These tend to produce threads which are collections of trivia, not the in-depth discussions about a particular topic we're looking for. If you have a specific question about a historical event or period or person, please feel free to re-compose your question and submit it again. Alternatively, questions of this type can be directed to more appropriate subreddits, such as /r/history or /r/askhistory.