If you look in the sidebar here (down towards the bottom), you'll see a heading that says 'Related Subreddits'. If you click on 'More', you'll be taken to a wiki page that lists many related subreddits; it includes a section for History subreddits.

You should also check out the sidebar of /r/HistoryNetwork - that's very comprehensive!

Obligatory plug for /r/Badhistory where We laugh at the worst history on Reddit and beyond. You will probably learn a lot about countering bad history too.

/r/HistoryOfIdeas is my second favorite history subreddit.