The occupying faction keeps forces in the field even when no battle runs. It posts patrols on the roads. It posts foot patrols in the towns it still holds. It keeps a garrison at each radio tower it controls. It builds up a defence at each base it holds over time. Base defences, vehicle patrols and tower garrisons all draw on one shared pool of resources. A faction short of money fields less of everything at once. A foot patrol in a town costs the faction nothing.
A soldier killed out of a patrol, a tower garrison or a base defence does not simply come back. The next time you see that unit it still carries only the men who survived the last fight. That holds true whether the unit walked out of sight and returned, or the whole campaign was saved and loaded again in between. A base, a tower or a patrol stays in the state you left it in.
A force wiped out to the last man is not gone for good. The occupying faction can rebuild it from nothing later, at a cost drawn from the same shared pool as everything else it fields.
A town patrol keeps walking its round even with nobody there to watch it. Come back later and it will not wait where you last saw it. It stands wherever along its circuit it has since reached. A patrol on a base perimeter behaves the same way.
A garrison holding a radio tower is different, and so is a group manning a base's fighting positions, towers or checkpoints. Those men hold their ground and stay put while nobody watches.
A base does not get its full defence all at once. It grows over the course of a campaign. A perimeter patrol and the first tower guards cost nothing and stand from the moment a base changes hands. Fighting positions, more tower guards, marksmen, checkpoints and fortified positions follow as the base earns the resources for them. Only once the occupying faction comes under real pressure does a base add a heavier patrol, an anti-tank team and parked vehicles. A base adds nothing new while a player stands inside it.
A base that has only just changed hands, or one short of money, can sit lightly defended for a while before it catches up. Losses at a base last the same way patrol losses do. The men who survived stand back on their posts the next time you look. A position wiped out completely can be bought again later. Taking the base itself is the only way to clear a post for good.
A radio tower changes hands only when every man garrisoning it is dead, and never before that. Standing near a tower does nothing by itself. Walking away and coming back does not reset a fight already under way. The garrison stays down by everyone already killed, however long you stayed away.
A tower can also sit with no garrison at all. A stretched occupying faction leaves towers bare. A tower you walked past unguarded may carry a fresh garrison days later, once the faction can afford one again. The occupying faction puts no new forces into the field while a battle runs anywhere on the island. The same is true while nobody is connected to the server. Forces already in the field carry on as normal. Only new ones wait.
See Counter Attacks for how the occupying faction can take a lost radio tower back, and Factions for the armed vehicles it puts on the roads.