One of the main features of Battle for Azeroth will not be available immediately upon the release of the new expansion of World of Warcraft. A community manager has confirmed this.
Warfronts will be available weeks after release: Community manager Ythisens officially confirmed in the WoW forums that warfronts will be available later. He did not specify an exact time, only that they would come “a few weeks later.”

Warfronts – no content for immediate access
Warfronts are one of the new features in Battle for Azeroth. Alliance and Horde are fighting for strategically important positions on Azeroth. The battle for Stromgarde is the first warfront you can visit.
Ythisens (via WoW forums) classified the warfronts as part of the “endgame content” alongside raids and rated battlegrounds – which is why they will only be playable a few weeks after the release of Battle for Azeroth.

The Alliance holds Stromgarde: Stromgarde will initially be held by the Alliance, as one of the strongholds in Lordaeron. The Horde will prepare for the siege and conquest in the first weeks of Battle for Azeroth.
When the time comes, you can sign up as a Horde player for the warfront and participate. You can take Stromgarde and hold the fortress until the Alliance completes its preparations. The battle is expected to cyclically shift back and forth.
Advantages for the conquerors: The faction that holds a warfront has advantages in the corresponding area for that time. For Stromgarde, this means that whoever holds the fortress has access to rare enemies and a world boss in Arathi Highlands.
Once control of Stromgarde passes to the other faction, they will gain access to world content in the area.

Launch possibly with the raid: There is still no exact date for when the warfronts will be available to players. However, it is possible that Blizzard will align their release with the new raid Uldir.
Uldir will open its doors on September 5.