Threats of Armageddon are flying between the U.S. and North Korea. President Donald Trump even implied he will launch a pre-emptive strike on North Korea, possibly using tactical nuclear weapons. The Pentagon has a plan to do just that. If carried out, however, it will fail and will likely undermine the real tools of arms control: sanctions and negotiations.

We have been here before. President George W. Bush used a Pentagon plan to attack Iraq pre-emptively in 2003, to prevent a “mushroom cloud.” The invasion led to a 14-year tragedy that is far from over.

President Bush could have avoided the Iraq catastrophe if he had followed a basic principle of international law. The United Nations Charter generally prohibits all use of armed force, except in two narrow situations. One is self-defense if an “armed attack occurs;” the other is with UN Security Council authorization if necessary to preserve the peace.

Lawful military force must always be a last resort and must have a high likelihood of success under the principle of necessity. Neither condition can be met in attacking North Korea. The Council just agreed to a tough new set of economic sanctions on North Korea. These have not had a chance to work. More importantly, military force has no track record of success in arms control.

Every success, from ending South Africa’s nuclear program to ending Saddam Hussein’s, has been accomplished through negotiations, often backed by sanctions. Negotiations nearly succeeded with respect to North Korea in 1994, but failed for reasons that could have been avoided. The positive reports of the Iran nuclear deal continue to roll in.

Some believe that Iran made concessions because of the threat of force by the U.S. and Israel. The more accurate view is that economic sanctions worked. If attacked pre-emptively, Iran would have had the right to counter-attack. Every sovereign state in the world is a member of the United Nations and understands the importance of the rule against the use of force. Iran would have had considerable support in responding to such an unlawful, pre-emptive attack.

Apparently, President Trump’s threats are based on the view that only fear of force will get North Korea to the bargaining table. This is a highly dangerous game that defies the evidence and undermines the very norms needed for a negotiated outcome. Pushed too far, North Korea may well unleash its own pre-emptive attack.

There may be only one international legal principle more important than the prohibition on the use of force, and that is the prohibition on the use of nuclear weapons. The International Court of Justice ruled in a case in 1994 that international law did not forbid five states, the Security Council’s Permanent 5, from possessing nuclear weapons.

But the World Court was equally clear that virtually no scenario exists where the P5 could lawfully use their nukes. Nuclear weapons have such a wide impact of killing and destruction they are inherently indiscriminate and disproportionate.

Officials are now downplaying Trump’s threats as just rhetoric. If so, then North Korea knows that, too, making them worse than useless.

Rhetoric alone can undermine the provisions of world order needed for an agreement to be negotiated and honored. The Security Council has come through with a tough set of new sanctions. The focus should be there, on unity, on building up the reputation of the Council and respect for international law so that mandates respecting North Korea are deemed worth honoring by North Korea and the global community.

Commentary by MaryEllen O’Connell, the Robert & Marion Short professor of law and is the
research professor of International Dispute Resolution, Kroc Insitute at University of Notre Dame University.

For the latest commentary on markets in the U.S. and around the world, follow @CNBCopinion on Twitter.