The Horde and the Alliance will take turns either invading or defending a zone on their continent. Neither continent will be attacked two times in a row, as each new incursion will be treated as a retaliation for the previous assault.
How to Locate an Incursion
While the continents are guaranteed to take turns in defending a zone. The zones themselves are chosen at random. To see if there is an Assault taking place, you simply need to open your map and to see the continent overviews. Any assaults will be displayed with are large faction icon, indicating the zone that is being attacked, and by which faction.
Assault World Quest Prerequisites
Any character that is level 110+ can take part, as long as they have completed the required prerequisite quest. If the Assault is occurring on your home turf. You are free to defend your continent from the invaders. Make your way to the zone and a breadcrumb quest will pop up for you to accept.
If the Assault is occurring on the other faction’s continent. Then you need to have begun the War Campaign, for that zone in order to be able to access the Assault World Quests. Once the the War Campaigns have been started, you will receive the pop up breadcrumb, allowing you to take part in the invasion of that zone.