The opening months of the second Trump administration have been characterized by a broad assault on the rule of law. Trump 2.0 has conjured a blizzard of pardons for political loyalists (e.g., January 6ers), retribution against law enforcement, threats of politically motivated prosecutions against prominent Democratic lawmakers, detaining dissidents for their speech, radical misuse of the Alien Enemies Act, the destruction of statutorily mandated federal agencies (e.g., USAID), purges of civil service personnel, announcements that statutes will not be enforced (e.g., TikTok ban), attempts to re-write the Constitution through executive order (birthright citizenship) or disregard the Constitution altogether (e.g., Appointment Clause and impoundments). This blitzkrieg against the rule of law has not been confined to actions within the borders of the United States.
Through both word and deed, President Donald Trump has also been aggressively undermining the rule of law internationally. This is particularly true with respect to two complementary legal pillars of the post-World War II international order: the prohibition on the use of force—particularly the ban on territorial conquest—and the United States’ obligations to defend treaty allies from armed attack. With respect to these areas of the law, the President and his administration have committed outright violations, enabled and seemed to endorse violations by others, and repeatedly raised grave doubts as to whether the United States would meet its legal obligations.
These actions are potentially deeply destabilizing, but unlike many of Trump 2.0’s domestic measures they are not amenable to meaningful judicial challenges. It will therefore be especially incumbent that executive branch lawyers and other officials attempt to check the President’s destructive actions in this sphere—as difficult and fraught as that might be. Such efforts will require more than simply providing legal advice when asked and instead necessitate proactive engagement with the President by his senior advisors—engagement to impress upon him why the United States helped establish these rules and how breaching them harms his own stated objectives. Members of Congress and the legislative branch as an institution should also push back against unlawful proposals from the White House, and though those efforts should be bipartisan given the Article I prerogatives being undermined by Trump’s actions, the minority may need to use the measures in their toolkit to do so if not joined by their colleagues in the majority.
The Prohibition on the Use of Force
As recounted by Oona Hathaway and Scott Shapiro, the United States took a leading role in drafting Article 2(4) of the UN Charter, which prohibits not only the “use of force against the territorial integrity or political independence of any State,” but also the “threat” of any such use of force. Although Article 2(4) is understood to proscribe any use of force, the core of the prohibition—flowing from the bitter experience of the Second World War—is the prohibition on conquest and threats of such actions. As I have previously argued, this prohibition on the use or threat of force is not only binding on the United States under international law, it imposes domestic legal obligations on the President individually, as the President has a constitutional duty to “take Care that the Laws be faithfully executed” and the “Laws” encompass treaties, specifically including the UN Charter.
In a manner without precedent for any U.S. President for at least the last hundred years, Trump has repeatedly issued public threats of annexation against a number of territories in the Western Hemisphere. Trump has vowed to “take back” the Panama Canal and the White House has reportedly asked U.S. Southern Command to provide military options, including seizing the canal by force. Trump has revived his ambition of acquiring Greenland (a territory of Denmark), recently proclaiming in an address to Congress that “[o]ne way or the other, we’re going to get it.” The President regularly refers to Canada as the “51st state,” particularly in the context of his trade war with the country, has repeatedly characterized the U.S.-Canada border as artificial, and reportedly has claimed the treaty demarcating it is invalid. The Canadian government at least views his stated desire to annex their country as sincere.
Although sometimes the threats are couched primarily in economic terms (e.g. tariffs), the potential for the use of force is often not so veiled. Others in his administration have also publicly threatened U.S. military action on Mexican territory against drug trafficking organizations.
It merits noting that the only exception to the non-use of force (or threats of force) aside from UN Security Council authorization is a legitimate invocation of national or collective self-defense. It should go without saying that there is absolutely no fig leaf of an argument in any of these situations that the United States has been attacked (or faces an imminent threat thereof) from any of these allies, partners, or organizations that could plausibly give rise to a lawful claim of self-defense.
Beyond the United States’ own threats of the use of force against western hemisphere neighbors, the Trump administration has facilitated or seemingly endorsed territorial incursion by others. Like his predecessor in the Biden administration, Secretary of State Rubio has continued to authorize arms transfers to Israel, notwithstanding the fact the Israeli military currently occupies territory in Lebanon and Syria without any apparent international legal basis. (The Arms Export Control Act requires that U.S. arms be used for “legitimate self-defense” which the State Department has in the past interpreted to mean consistent with the UN Charter.)
More notoriously, the Trump administration has reversed the United States’ strong diplomatic and material opposition to Russia’s illegal invasion of Ukraine by temporarily pausing U.S. military support, voting against a UN resolution condemning Russia, and appearing to hint that Ukraine may need to cede territory to Russia, with the President reportedly considering U.S. recognition of Russia’s unlawful annexation of Crimea.
U.S. Defense Commitments
Complementing the prohibition on the use of force are U.S. legal commitments to allies to defend them should they be victims of armed attack. The most prominent such mutual defense agreement is the North Atlantic Treaty, Article 5 of which provides, in pertinent part that:
The Parties agree that an armed attack against one or more of them in Europe or North America shall be considered an attack against them all and consequently they agree that, if such an armed attack occurs, each of them, in exercise of the right of individual or collective self-defence recognized by Article 51 of the Charter of the United Nations, will assist the Party or Parties so attacked by taking forthwith, individually and in concert with the other Parties, such action as it deems necessary, including the use of armed force, to restore and maintain the security of the North Atlantic area.
Although Article 5 does not require any specific action by the parties and Article 11 of the treaty specifies that its provisions shall be carried out in accordance with the constitutional processes of the respective parties (e.g. the U.S. Congress may need to authorize the use of force), Article 5 nonetheless imposes an obligation to assist its NATO allies if they are attacked.
Article 5 has notably been invoked once—following the 9/11 attacks on the United States—and NATO members immediately and over the next several decades took a range of actions in collective self-defense of the United States, up to and including committing troops to active combat. NATO allies responding in the collective self-defense of the United States included both Canada and Denmark, recent targets for President Trump’s threats.
The United States’ mutual defense treaties with Japan and South Korea contain somewhat different obligations to “act to meet the common danger in accordance with its constitutional [provisions and] processes” in the event of an armed attack on one of the parties.
Although the President (and Congress) retain substantial discretion as to how the United States will respond in execution of its mutual defense obligations, refusing to act at all or seeking to unilaterally condition any U.S. response are inconsistent with the United States’ legal commitments.
But during both his first term and in the early weeks of his second administration, Trump has raised serious doubts as to whether the United States would in fact come to the defense of a NATO ally, Japan, or South Korea, if those states were attacked. Recently he suggested that the United States would not defend NATO allies from attack unless they spent 5% of their respective GDPs on defense. President Trump also recently criticized the U.S.-Japan defense treaty and in 2024 during the presidential campaign suggested he might remove U.S. forces from South Korea as President. (During his first term, Trump reportedly risked war with North Korea by almost threatening to withdraw U.S. dependents from the peninsula while his administration tried to drive a hard bargain with Seoul over financial support for U.S. forces.)
Why It Matters
Although the prospects of Trump backing up his threats with the actual use of military force may be remote (with the significant potential exception of military action in Mexico), the words of the President of the United States still matter.
First, as noted above, threats of the use of force by themselves breach Article 2(4) of the UN Charter and are inconsistent with the President’s duty under the constitution’s Take Care Clause. This is not a minor infraction—the non-threat or use of force is considered the bedrock of the international system for a reason: it protects all States, large or small, from the anarchic chaos and enormous human consequences of wars of conquest (or of being coerced into undermining their own interests under threat of such a war).
Second, the United States has long held a uniquely powerful role in the international system, due to the strength of its armed forces and global footprint in the post-World War II era. Coming from the commander in chief of the world’s most powerful military, such threats are deeply corrosive to the prohibition on the use of force.
Third – from the perspective of U.S. interests – blustery talk of military action combined with wielding of tariffs inflicts gratuitous damage to bilateral and regional relations with targeted partners and allies. This damage imposes real, tangible costs on the United States. Threats against allies undermines the perceived reliability of the United States as a provider of military support, leading other countries to seek alternative sources. The concrete costs of such hedging may be losses of the very sorts of manufacturing jobs President Trump claims to want to bring back to the United States and weakening the U.S. military-industrial base. More generally, such talk torches U.S. soft power resulting in the people of targeted allies and partners less likely to buy U.S. goods or travel to the United States—economic ramifications that ought to matter to the President.
Fourth, such talk may embolden other states—most notably Russia—who may harbor designs on the territories of their neighbors.
Fifth, there’s also a potential consequence of the weakening of U.S. defense commitments by Trump that he himself may care about on a personal level—the increased risk of nuclear proliferation. Since the 2024 presidential election, multiple U.S. government non-proliferation experts have emphasized to me that Trump’s return to office significantly increased the likelihood that a number of U.S. allies would pursue nuclear weapons as a hedge against the diminished reliability of U.S. extended deterrence. Two U.S. officials I spoke with recently identified South Korea, Japan, and Poland as the US allies most likely to seek the nuclear arms given a second Trump administration. In early March, Canada’s former foreign minister, Chrystia Freeland, (then a contender for Prime Minister) suggested that Canada should seek extended nuclear deterrence from France or the United Kingdom—deterrence against the United States—in light of Trump’s persistent rhetoric about making Canada the 51st state.
In short, the United States behaving as a predatory hegemon unconstrained by the UN Charter and threatening other states with territorial conquest— in many cases states that previously relied on the United States to protect them against threats of this nature — may spur more countries to acquire their own atomic insurance policies.
There is a particularly grim irony to Trump goading nuclear proliferation. Since at least the 1980s, Trump has styled himself as an expert on non-proliferation, even proposing a “Trump Plan” involving the United States and Soviet Union working jointly to pressure other countries with nuclear ambitions (such as Pakistan at the time) to foreswear them. In early March, Trump told reporters that:
“[It] would be great if everybody would get rid of their nuclear weapons… It would be great if we could all denuclearize because the power of nuclear weapons is crazy.”
Trump’s erosion of the prohibition on the use of force and U.S. defense commitments will only undermine this stated ambition.
Speak Law to Power
President Trump appears to believe that the United States is ill-served by the constraints and obligations of the post-World War II security architecture that the United States played a critical role in establishing. He may have good reason to believe that decades old U.S. defense alliances should be reviewed, updated, and burdens rebalanced. But bluntly calling into question whether the United States would in fact fulfill its mutual defense commitments undermines the credibility of any future such alliances and works contrary to Trump’s very own publicly stated goals. As does encouraging invasion and conquest through the violation and erosion of the prohibition of the use of force and threats of the use of force.
Executive branch lawyers and other U.S. officials need to speak “law to power” to the President on this front, including aiding him in executing his constitutional duty under the Take Care Clause. This will mean more than merely providing candid advice when asked. It will likely require the President’s most senior advisors such as the Secretary of State, National Security Advisor, and White House Counsel to proactively intervene to explain to the President why the United States voluntarily assumed these obligations in the first place and what purpose they continue to serve.
Providing such advice will be challenging, with both senior officials and administration lawyers likely fearing the consequences of delivering unwelcome guidance. Members of Congress should also push back against these unlawful proposals (following the lead of Representative Castro (D-TX)), including to reassert the legislature’s constitutional prerogatives on war powers.
Certainly previous efforts to educate Trump on the post-Second World War order the United States helped create have not always gone well. But it is necessary. The commander in chief should understand that bellicose threats and perceived unreliability as an ally may leave the United States poorer and more vulnerable on his watch, and will frustrate his own articulated ambitions with respect to nuclear nonproliferation.
– Brian Finucane (Bluesky – LinkedIn) is senior adviser with the U.S. Program at the International Crisis Group and a Non-Resident Senior Fellow at the Reiss Center on Law and Security at NYU School of Law. Prior to joining Crisis Group in 2021, he served as an attorney-adviser in the Office of the Legal Adviser at the U.S. Department of State. Published courtesy of Just Security.
