Except by that definition, the unleashing of nuclear weapons by the US on Britain tomorrow without provocation would be to protect its' own self-interest. Nevermind the obvious side effects and other factors at play. There is a reason why there are limits to what pure power or possession can bring for a state in terms of even its' own self interest. .... Except quite frankly it does not always. It may seem to be, and when seen with certain lenses we can even imagine how it seems to be, but that does not change A: the validity of an average cost-benefit analysis, B: actions throughout history that frankly were visibly against the self-interest of the states involved (like Napoleon's overthrow of his Spanish ally, to give but one reason), and C: the other issues involved beyond self-interest. Endlessly repeating those canards cannot and will not change the fact that it is not always so.
Self-interest may be objective, but our perception of it absolutely is not. Just like our perception of morality is also subjective, and it is to these subjective perceptions (amongst other matters) that states act, not to the cosmic, objectively true ultra ultra facts which we as humans cannot fully have but which we can to some level discern. Treating geopolitics or human nature like it is some laboratory experiment in the eye of the beholder is to ignore the entire benefit of studying either or what they can tell us.
The red herring is straitjacketed, linear, and dogmatic thinking that holds that to some degree we *are not* discussing social psychology here, or that we can discuss the actions of the state without doing so. For all your eloquent words, you seem remarkably unable to come up with explanations or facts for historical events- the gold standard by which events in our world can be analyzed- in which matters other than reactions to (perceptions of) self interest were in play.