Ebbtide's campaign is built around one persistent resource: the people you rescue. Everything else - the hardware, the bases, the map control - exists to protect and expand that force.
The campaign opens in zombie-overrun Los Angeles with the front line running from the Hollywood Hills down to the Port of Los Angeles. That span is the map, and it is where you learn the whole loop: establish a foothold, fortify it, open a supply line, and start pulling survivors out.
Trucks and helicopters pull survivors out of enemy-held territory, and rescued survivors become the starting force for later regions. Losing them costs more than losing any single fight, so extraction routes deserve the same protection as your front line - and should be positioned behind it rather than alongside it.
Resupply comes by truck and helicopter, and the route lengthens every time you push forward. Long routes are long thin targets, and the infected will find them. Plan the logistics before you plan the conquest - a forward base with no convoy behind it is a temporary position at best.
Once the Los Angeles theatre is stable, you take the carrier to the next region and hold new ground, with regional defence missions in each. The escalation runs from city-scale street fighting to a global campaign, and each new region adds a different problem rather than just more infected.
This walkthrough outline is inferred from the officially documented game loop - Ebbtide is unreleased and no hands-on walkthrough exists yet. The structure above follows what the developer has stated about the opening theatre, the extraction system and the World Campaign, but the specific order in which you will meet these systems in the final game may differ.