The discourse on "security policy" lacks a certain clarity of thought and clarity of purpose in my opinion. It's very much possible and even outright likely that resources will be wasted, avoidable hostilities be created, wealth, health, freedom and lives be lost if nations follow a path in "security policy" without such clarity of thought and clarity of purpose.
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Humans live in groups because this is mostly the better way to live, and as groups they communicate and deal with other groups through representatives. Much of this is engrained in our genetic heritage apparently, but we have evolved our groups quickly during the last couple thousands of years. Clans were replaced by tribes, and tribes were replaced by states. States have created multi-state groups - even alliances with their own bureaucracy. Our instincts did not match this development because it was too quick. Much of our instincts suits the hunter-gatherer band much better than it does a modern state. Thus we need to think with great clarity and overrule our instincts often in order to achieve better outcomes.
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Defence policy is first and foremost about warding off the loss and suffering brought about by war on one's own soil. It extends to warding off as much of the loss and suffering brought about by war in general - even if war was forced onto the state.
Security policy follows a broader definition. It extends as far as influencing the threat and outcomes of war in distant places, in an attempt to garner advantages for oneself. Said "oneself" is never the whole population or citizenry of one's state, but typically a special interest group or conglomerate thereof.
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Almost no group has ever been able to defend itself or will ever be able to reign do so without the tolerance by other groups. Even great powers thus seek cooperation with or allegiance of other states, supra-state institutions and even non-government groups.
Some weaker states sense threats to themselves which don't believe to be able to ward off with their own strength. Others believe this to be possible, but still consider cooperation with other states to be more favourable.
These states have good reasons to look at alliances as an institution which enables their defence policy to accomplish its mission, and typically so for much less effort than if defence policy was done in isolation. A state which believes itself to be able to defend itself and still enters an alliance would be led by fools if the resources allocated to defence policy wouldn't be reduced ceteris paribus to match the improved situation.
Weak states' leaders in a strong alliance are exposed to the same temptations as the leaders of strong states: The temptations to use the power at hand for policies which don't provide the public good of security against war's losses and sufferings, but advantages to special interest groups. Said special interest groups could even be foreign groups, such as (supposedly) ideologically aligned groups. It could also be a single ruler himself, driven by a taste for playing games with the game pieces available to him or her.
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"Security policy" as well as "defence policy" could also be defined on basis of misconceptions about the outside world. Pre-historic hunter-gatherer groups may have misjudged the intentions of another group they met. Today's politicians, lobbyists, pundits, scholars, flag officers and interested citizens could easily misunderstand a foreign group's motives, capabilities, actions and plans. Ignorance about expenses required or benefits gained by certain policies usually leads to poor judgements as well. More general forms of incompetence aren't rare either.
The result could be wasteful and the actions could self-defeat their purpose - even without any pattern of anti-social motivations on part of the actors.
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Changes in the status quo are more impressive than the status quo and affect our judgement out of proportion. Arriving reinforcements influence soldiers' morale in battle much more than their participation right from the beginning of the battle, for example. Such judgement suitable to bands of hunter-gatherers is ill-suited for decision-making on a state or alliance level. We know people react based on pre-historic patterns and should take this into account regarding the how to do, but not in decision-making regarding what to do. A politician serves his people best if he decides on a course of action without undue influences, but recognises that he needs to take into account undue influences on others in order to succeed.
Back to changes in the status quo; a change in the relative economic or military power usually provokes exaggerated reactions, which can easily lead to avoidable losses and suffering. It is most important to preserve, gain or regain clarity of thought and purpose when one faces events which outright provoke a breakthrough of the primitive self.
Avid readers of international news on security policy or history can easily find many instances of hysteria, irrationality, wastefulness, misjudgement unnecessary hostility and incompetence. These are failures of outgrown hunter-gatherer groups to react with clarity of though and clarity of purpose to their outer world. People suffer and die in the process. Anyone who tries to reduce such failures has a noble goal in life.