Steven Metz considers the implications of a preventive war launched by the U.S. against North Korea or Iran:

A preventive attack against North Korea or Iran, though, would be the death knell for the international law of armed conflict. When a small nation like Israel launches preventive attacks, such as the 1981 strike on Iraq’s Osirak nuclear facility, the international legal system is bruised but survives. If the United States openly violates the prohibition on preventive war, the international law of armed conflict will become meaningless. No nation will feel bound by it.

I agree that launching a preventive war against either country would be a flagrant violation of international law and would undermine it severely, but I’m not sure it would be “the death knell” that Metz describes. For all intents and purposes, the U.S. already launched a preventive war in 2003, but that has not yet prompted other states to feel free to imitate the same behavior. The invasion of Iraq was sold as “pre-emption,” but even if every administration claim about Iraq’s weapons programs had been true there was nothing “pre-emptive” about attacking Iraq.

Bush and his officials specifically rejected the view that the U.S. could or should wait until there was an imminent threat. He memorably said, “If we wait for threats to materialize, we will have waited too long.” Bush was making an argument in favor of preventive warfare while continuing to call it pre-emptive: “And our security will require all Americans to be forward-looking and resolute, to be ready for preemptive action when necessary to defend our liberty and to defend our lives.” Later in 2002, he infamously made this statement:

Knowing these realities, America must not ignore the threat gathering against us. Facing clear evidence of peril, we cannot wait for the final proof — the smoking gun — that could come in the form of a mushroom cloud.

When the U.S. invaded Iraq, it wasn’t preempting an imminent threat. According to Bush’s own claims, the U.S. was attacking before the (imaginary) threat could develop. Iraq was a preventive war. My point here is that the U.S. already ran roughshod over international law back then, but fortunately almost all states still respect the prohibition that we blatantly flouted.

Aside from the enormous costs and dangers of starting unnecessary wars, the U.S. should repudiate preventive warfare because it is both illegal and inherently unjust.