Here’s my view of why countries don’t attack each other: The downside for the attacker has to be larger than the upside, that’s when diplomacy becomes interesting. The downside doesn’t just have to be the defense of the attacked country but also the relation with other countries. The US won’t invade Australia because they wouldn’t gain much, compared to the loss of trust by other countries. Defense from china is more important than from the US for Australia, because the “public stage” deterrent is smaller for china than the US. That’s why you need to increase the deterrent by increasing your defense capabilities. You can correct me if you disagree though.