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John B. Bellinger III is an adjunct senior fellow for international and national security law at the Council on Foreign Relations.
President Donald Trump called for Iran’s “unconditional surrender” in a social media post on Tuesday and claimed that “we now have complete and total control of the skies over Iran.” These statements follow reports that the White House is considering joining Israel’s military campaign against Iran’s nuclear program and other military targets.
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This possibility has raised concerns about presidential authority and the role of Congress in approving the use of military force overseas. Lawmakers have introduced resolutions in both chambers on Wednesday that would require Trump to obtain congressional approval before U.S. service members could participate in any offensive operation against Iran. Despite these potential constitutional issues, Trump did not walk back his threat, telling reporters that “I may do it. I may not do it. I mean, nobody knows what I’m going to do.”
Here is what the law and U.S. Constitution say about presidential authority and why it is relevant to U.S. involvement in Iran.
Trump has threatened to attack Iran. What authority does the president have or need to launch such a strike?
Whether President Trump has legal authority under U.S. domestic law or international law is highly debatable. As I explained in a testimony [PDF] to the Senate Foreign Relations Committee in 2017, when President Trump was threatening the use of force against North Korea, the president has broad authority under the Constitution to order the use of military force.
His Article II powers include authority not only to order the use of military force to defend the United States and U.S. persons against actual or anticipated attacks, but also to advance other important national interests. Presidents of both parties have deployed U.S. forces and ordered the use of military force, without congressional authorization, on numerous occasions.
In addition to the powers granted to the president in Article II, Article I of the Constitution gives Congress the authority to “declare War.” But this authority has never been interpreted—by either Congress or the executive branch—to require congressional authorization for every military action that the president could initiate.
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Would Trump have to obtain congressional authorization to use the U.S. military to strike Iran?
In several opinions, the Department of Justice’s Office of Legal Counsel (OLC) has acknowledged that the “declare War” clause may impose a potential restriction on the president’s Article II powers to commit the U.S. military into a situation that rises to the level of a “war.”
The OLC has stated that whether a particular planned engagement constitutes a “war” for constitutional purposes “requires a fact-specific assessment of the ‘anticipated nature, scope, and duration’ of the planned military operations.” It added that this “standard generally will be satisfied only by prolonged and substantial military engagements, typically involving exposure of U.S. military personnel to significant risk over a substantial period.”
Applying these principles to an attack on Iran, if the use of military force would be substantial and prolonged or would pose a substantial risk to U.S. forces or American civilians—such as from attacks by Iran or its proxies against U.S. interests—there is a good argument that it would require congressional approval consistent with Congress’ authority in Article I to “declare war.”
With regard to whether or not congressional authorization is required as a matter of law, both Republican and Democratic presidents have generally preferred, for political and legal reasons, to seek congressional authorization—or say that they are acting under previous authorizations—for any substantial or prolonged use of military force. President George H.W. Bush sought and received congressional authorization for the Gulf War in 1991, and President George W. Bush sought and received authorizations in 2001 and 2002 to use force against the perpetrators of the 9/11 attacks and against Iraqi President Saddam Hussein.
If Trump decided to proceed, would a U.S. attack against Iran violate international law?
Many international lawyers would argue that a U.S. military attack on Iran under the present circumstances would violate international law, although an analysis will depend on the facts of any such action. The UN Charter and customary international law prohibit the use of force against another country except in cases of self-defense or collective self-defense, or if authorized by the UN Security Council.
The right to self-defense includes the right to use force to defend against an imminent armed attack. The United States has taken a broad view of “imminence” in cases of threats of terrorism or mass destruction, but it would be hard to argue that a U.S. attack against Iran’s nuclear complex or leadership would constitute an act of self-defense against an imminent armed attack by Iran on the United States.
Instead, the Trump administration might argue that an attack on Iran constitutes an act of collective self-defense of Israel. The validity of this argument would depend on whether Israel acted in accordance with international law in attacking Iran in the first place and whether U.S. use of force is limited to protecting Israeli civilians and U.S. interests from Iranian attacks.
It is worth noting that in 1981, President Ronald Reagan’s administration joined a UN Security Council resolution that unanimously condemned Israel for launching a surprise attack on the Osirak nuclear reactor in Baghdad. The resolution stated that “diplomatic means available to Israel had not been exhausted.” In 2007, the George W. Bush administration declined an Israeli request to participate in destroying a nuclear reactor in Syria, fearing that it would further destabilize the region after the Iraq War. Of course, neither the Iraqi nor Syrian regimes posed as serious a threat to Israel and regional security as Iran does today.
If the president does not go to Congress, how would that expand presidential authority and potentially stress the U.S. system?
Over the last two decades, Congress has acquiesced more and more to uses of military force by presidents of both parties without congressional approval and with little congressional oversight. Although the president has broad authority under the Constitution to use military force to protect and advance American interests, an attack on Iran would be a very significant stretch of that authority and would certainly create significant risks to the U.S. military and U.S. citizens.
If Congress is to have any role in war powers, members of both parties should insist that the president consult fully with Congress before any use of military force against Iran.
This work represents the views and opinions solely of the author. The Council on Foreign Relations is an independent, nonpartisan membership organization, think tank, and publisher, and takes no institutional positions on matters of policy.