Mira Rapp-Hooper spells out why a “bloody nose” attack on North Korea would be so disastrous:

Given the staggering costs of a U.S.-led conflict on the Korean Peninsula and the difficulties North Korea would face in trying to upend U.S. alliances to conquer the South, the Trump administration’s use of preventive force would be a suicidal reaction to uncertainty [bold mine-DL]. This would not be a case of choosing the least bad option, as Jeffrey argued, but of opting knowingly for cataclysm.

It is worth repeating that launching an attack on North Korea would be illegal and unjust. If the U.S. carried out such an attack, it would be guilty of waging aggressive war in direct violation of the U.N. Charter. The U.S. would not be acting in defense of itself or any other country, but would be initiating hostilities in the (vain) hope of intimidating another state into acceding to its demands. An attack on North Korea would be a flagrant violation of international law. Preventive war cannot be waged as a last resort, so it is inherently unjust. In this case, the use of force is also certain to cause greater evils than it prevents. Starting a war with North Korea would have terrible consequences and should be avoided for that reason alone, but it would also be profoundly wrong for other reasons.

Preventive war has come to be treated as a debatable and therefore potentially acceptable policy option ever since 2002-03. This has been one of the more harmful and enduring effects of the Iraq war. Opponents of attacking North Korea absolutely should call attention to the calamitous effects that an attack would have, but they should also make the repudiation of preventive war in principle a central part of their arguments against any attack.