Wars used to be fought for a greater reason. Maybe for justice, maybe for power. One or more powerful guilds just starts to hunt another powerful guild until the members pay to the other guild's leader or disbands the guild.
Wars could take several weeks or months to end, at the end of the war lots of players had red skulls depending on what side is more agressive. Between great wars that involves a large part of the server, you will often find periods of peace. But sadly, wars go on and heroes just like this day just for walking around.
Before Update 8.5 almost every Open PvP worlds used to have wars. Then, high skilled guilds used to offer themselves to fight wars for others. Players shared their account with those high skilled players to win wars. Keep in mind that it's an illegal decision and can make you get banished.
But after Update 8.5, the new Skull System ended wars. Now wars are fought just for fun. You declare war against the other guild, which can accept or not the declaration. You can decide how long the war will takes, how many kills and how much you or the other guild will pay to the winner.
One of the first wars took place in the early 2000's on the server of Trimera. Several players joked about starting a war. From this joke it turned into a lot more players starting a war between the city of Thais and the city of Carlin. This was also before the skull system so there were a large number of players being killed over a joke. This war also was fought by players luring Giant spiders to either and both cities. At this point the monsters didn't disapear after being moved a certain number of squares. There also were not the bridges preventing the luring of monsters to cities.
See also: War System.