The Security are the defenders - they really don't seem to be fighting for anything, and are more just trying to stop the Resistance from achieving their goals.
this seems to be a common contemporary way of thinking for some reason- that obstructionism has no real value other than denial. Obviously they are fighting to achieve a goal, even if it
is just to prevent the Resistance from achieving their goals.
I reject that premise, though. It's pretty clear that they're fighting to protect resources, and also some more philosophical ideals, if the "preserve the Founders' vision" objective means anything.