This page describes an active game mechanic. See also QRF for how the fighting itself is scored, and Ruins and Repair for what sabotage against a base you hold does.
The occupying faction does not only defend. Once it holds enough in reserve, it can pick one town or base you hold and mount a slow campaign to take it back. Only one such campaign runs at a time, on top of any battle you start yourself.
The campaign runs in three stages: harassment, a forward base, and a siege. Nothing locks the objective in early. Losing the town or base to someone else, or dropping its support below the trigger level, ends the campaign there.
The occupying faction starts by sending small teams against the objective. It sends them more often on harder difficulty settings. A team can hold a town centre to push its support down, try to retake a nearby radio tower, or slip into a base you hold and wreck a structure there. Fight a team off, or just stand your ground near it, and the clock stops. The attempt then starts again from the top. It does not pick up where it left off.
A base a team wrecks is not lost for good. It leaves a ruin you can repair for money, and anything stored inside survives untouched. See Ruins and Repair for what a ruin can and cannot do.
Once harassment has pushed hard enough, the occupying faction raises a forward base near the objective. This is an unannounced camp that reinforces the coming siege. You can find it and tear it down by hand, at a cost to the occupying faction. It can also starve on its own. That happens if the base supplying it falls, if its garrison dies, or if you hold the ground around it in strength for long enough.
The final stage only opens during the day. Troops walk in quietly and take positions in a ring around the objective. Nothing shows on your screen while this happens. Once the ring closes you get a warning, a ring on the map, and a countdown of fifteen real minutes before the fighting counts for anything. Wipe out every group in that ring before the countdown ends, and you win on the spot. You do not have to wait out the rest of the window. Otherwise, once the countdown ends the siege turns into an ordinary battle for the objective, scored the same way as any other QRF.
If you beat off the siege, the occupying faction tears down what is left. The whole campaign can start again later, against a new objective.