“theory of action”: you work forwards from what you know how to do to try to find things you can do that will accomplish your goal.
A theory of change is the opposite of a theory of action — it works backwards from the goal, in concrete steps, to figure out what you can do to achieve it. To develop a theory of change, you need to start at the end and repeatedly ask yourself, “Concretely, how does one achieve that?” A decrease in the defense budget: how does one achieve that? Yes, you.