How do you have a war in Lord of the Rings online?

Answer 1

The game is set against the background of the War of the Ring; many of the tasks the player does are intended to either aid the Fellowship of the Ring directly or to hinder the Enemy generally.

If you mean "have a war against other players" ... that would be incredibly foolish when the Dark Lord is gathering power.

There is a "Monster Player" option for subscribers, who can play the part of monsters and fight against other players in PvP combat. You have to be a subscriber in order to participate in this, though, if you're a free-to-play player it's not an option.