As tensions continue to escalate with North Korea after their most recent missile tests, President Donald Trump is considering using military force against North Korea. Reportedly, the president intends to act before North Korea’s advancing missile technology and nuclear weapons capacity pose a direct threat to the mainland United States. Of course, this decision has significant military, diplomatic, economic and political considerations and consequences, but the Trump Administration will first have to determine if such an act would even be legal.
The answer to this question has two parts. First, under U.S. law Congress has power to declare wars and authorize funds to support them. The president would therefore be in the strongest legal position if he obtained congressional approval before initiating any military force against North Korea.
However, the president may use force without congressional approval if responding to an attack against the United States or U.S. forces. Thus, if North Korea attacked any part of the United States or U.S. forces, including those stationed in South Korea, the president would have constitutional authority to use force in response.
The president also has temporary authority to use force for up to 60 days before obtaining congressional authorization. Even if North Korea did not strike first, the president might still have authority to use force temporarily to prevent North Korea from attacking the United State in the future. But Congress would want concrete answers on how long the conflict was expected to last, what resources would be required and what the strategy was before providing longer term authorization.
Second, under international law, to which the United States is party, states are prohibited from using force against other sovereign states. A state may only use military force against another state in self-defense or with UN Security Council authorization.
The latter is extremely unlikely, as Russia and China would almost certainly use their vetoes to prevent force from being used by the United States. The former is a more complicated question and will depend on the precise facts on the ground. Of course, states do not need to wait until they are actually attacked in order to act in self-defense — this would make little sense in an age where weapons of mass destruction could destroy or devastate entire countries. But a state cannot use force at mere potential threats.
It is generally accepted that if a state faces an imminent, unprovoked and certain threat, it may use force in anticipation of that attack. Less accepted is a state’s use of “preventive” or “pre-emptive” self-defense — when a state attacks another it perceives as a threat, but before that threat becomes imminent or certain.
While North Korea certainly poses a threat to the United States and its allies in the region, that threat has not likely matured in its imminence or certainty. That could quickly change, but unless North Korea does more to demonstrate its ability and intent to imminently strike the United States, international law will prohibit the use of preventive force against it.
As a result, the answer to the question of whether President Trump may legally use military force against North Korea is mixed. Under U.S. law the president would likely have authority to use force as a temporary measure, but under international law he would be prohibited from using force at all. The administration would thus be wise to ensure alignment of these answers before taking any forceful action.
Ryan Vogel, JD, LLM, is the director of National Security Studies at Utah Valley University and assistant professor of law and national security. He previously served in the Office of the Secretary of Defense at the Pentagon.