This will become more important as technology decreases the difficulty of building WMDs. Eventually, even a small nation like North Korea will be able to make nuclear missiles. By that time, the cost of allowing them to do as they please (and encouraging other nations to also do as they please) may be greater, in expected lives lost, than the cost of brutally killing a million North Korean civilians.

Naturally, the North Koreans will use similar reasoning just as hard as they can. How powerful do you need to be before you are promoted from 'pre-emptive strike' to 'cold-war'? (I support placing N. Korea in the former category for what it is worth.)