Forum Wars were thread wars that usually consist of a forum poster posting a thread in a not-liked forum saying their forum is attacking. It ends with big amounts of spam threads and bans, due to forum moderators not being fond of them. It was one of the many reasons why the Forums were removed on December 11, 2017.
Forum wars were usually in the Let's Make a Deal forum VS the Clans & Guilds forum due to those two forums thinking they are better than each other. The posts from C&G were usually "We know how to fight and you guys love your pixels" and the posts from LMaD were usually "We can buy C&G".
Lesser-known forum wars were usually against the roleplaying forum, oftentimes criticizing it due to the behavior of players in roleplaying games, rather than the far more professional roleplayers in the forum.
An example of a player, who's name is Clanology, starting a forum war.