Six months after Congress expressly disapproved US military action in Yemen, the war continues unabated. Congress has additional tools available to restrict or end US involvement. But as Congress’s impeachment inquiry moves forward, there’s another question: Is President Trump’s ongoing prosecution of military action in Yemen in defiance of congressional opposition a ground for impeachment?

The Constitution’s War Powers Clause grants Congress, not the president, the exclusive power to decide whether to engage in warfare. To be sure, the president is “Commander in Chief of the Army and Navy.” However, constitutional history — reflected in the Framers’ deliberations, the Federalist Papers, debates in state ratifying conventions, the writings of James Madison, and more — confirms that the Framers intended to empower the president “to repel sudden attacks” and to command military forces in offensive wars, but not to decide whether to engage in them. As Leon Friedman and Burt Neuborne have noted, “There are areas where the intentions of the Founders are ambiguous. In matters of war and peace, however, they could not be clearer.”

But is unauthorized presidential warfare impeachable? In his 1974 classic Impeachment: A Handbook, Charles Black described unauthorized military action as “the most agonizing question of all.” He observed that “[a]s a new matter, I should have thought that totally unauthorized entrance into hostilities, without any emergency or any immediate threat to the nation, was the grossest possible usurpation of power, clearly impeachable.” Yet he ultimately concluded that “only a very extreme and not now visible case ought to bring the impeachment weapon into play as a sanction against presidential warlike activity.” Does Trump’s ongoing pursuit of the war in Yemen meet that test?

I. Impeachment and unauthorized military action

A. History and precedent

Based on historical practice, the Nixon impeachment inquiry’s landmark report on Congressional Grounds for Presidential Impeachment divided impeachable offenses — what the Constitution calls “high Crimes and Misdemeanors” — into three broad categories, starting with “exceeding the constitutional bounds of the powers of the office in derogation of the powers of another branch of government.”

History teaches that unauthorized warfare fits into that category. As far back as 1682, Maryland’s colonial legislature impeached and convicted an official for undertaking an unauthorized war against the Piscataway Indians. But the most salient example was contemporaneous with the Constitutional Convention in Philadelphia. In 1787, just weeks before the convention opened, the British House of Commons voted to impeach Warren Hastings, the former Governor-General of India. Reports of Hastings’s impeachment circulated widely in North America. And notably, the very first article of impeachment against Hastings charged that he violated instructions not to engage in “any offensive war whatever.”

As the Framers debated the wording of the Constitution’s impeachment clause, George Mason cited the well-known charges against Hastings to illustrate why impeachment should cover more than just treason or bribery. As Mason explained, treason would not cover “many great and dangerous offences,” noting as an example that “Hastings is not guilty of Treason.” To cover the “great and dangerous offences” illustrated by the charges against Hastings — which, again, began with unauthorized war — Mason eventually proposed the phrase “other high crimes and misdemeanors.”

Just seven years after the Constitution’s ratification, the young nation’s very first impeachment case confronted the issue. In 1797, the House voted to impeach Sen. William Blount for conspiring to conduct an unauthorized “military hostile expedition” against Spanish territories. (The Senate expelled Blount, but dismissed impeachment charges on the grounds that senators are not “civil Officers” subject to impeachment.)

The closest that Congress has come to impeaching a president for unauthorized military action involved Richard Nixon. In 1974, the House Judiciary Committee considered — but decided against — an article of impeachment for unauthorized (and secret) bombing of Cambodia. In the committee debate, Rep. Don Edwards (D-Cal.) explained that impeachment would help re-establish a critical constitutional principle: “[I]t is time to send the message to future Presidents and this President about undeclared wars. Congress alone has the power to declare war. The President does not have the power, and it is an impeachable offense to wage an undeclared war.” However, the article was defeated 12-26 in committee.

Finally, even though no president has yet been impeached for unauthorized warlike activity, federal courts have on multiple occasions cited the availability of impeachment as a reason to treat litigation over the War Powers Clause as non-justiciable.

B. Arguments against impeachment for unauthorized military action

Black identified four main objections to impeaching a president for unauthorized warlike activity, and they deserve consideration.

First, Black noted that previous presidents had engaged in unauthorized military operations, such as the Bay of Pigs invasion, that did not result in impeachment.

Second, he observed that Congress sometimes sends mixed messages regarding authorization. That was certainly a major factor in the Judiciary Committee’s decision to reject an article of impeachment against Nixon for the Cambodia campaign. As the committee report summarized, “opponents of the Article concluded that, even if President Nixon usurped Congressional power, Congress shared the blame through acquiescence or ratification of his actions.”

Third, Black noted the difficulty in separating the constitutional issue of a war powers violation from opinions about the military action itself. Referring to the (then recent) Yom Kippur War of 1973, he rhetorically asked: “[W]ould it be thought that an impeachable offense had been committed if our forces in the Mediterranean were ordered to intervene to keep the Syrians from taking Haifa?” Black concluded that “the wrongness of unauthorized military action is likely to seem clear, on the whole, only to those who disapprove substantively of the particular intervention.”

Finally, Black emphasized the importance of alternative remedies, memorably writing that Congress need not “sit idly by, counting up grievances, until time comes to call a council of elders and sharpen the impeachment spear.” Rather, Congress has a wide range of tools which, Black believed, should be tried before impeachment. For example, Congress could refuse to appropriate funds for prosecuting an unauthorized war, or pass a concurrent resolution “declaring in unmistakeable terms that the war was immoral and against the country’s interest.”

II. The case of US military action in Yemen

In the spring of 2019, Congress passed an unprecedented joint resolution, S.J. Res. 7, by votes of 54-46 in the Senate and 247-175 in the House. The resolution provides that “Congress hereby directs the President to remove United States Armed Forces from hostilities in or affecting the Republic of Yemen, except United States Armed Forces engaged in operations directed at al Qaeda or associated forces, by not later than the date that is 30 days after the date of the enactment of this joint resolution.”

Here the matter gets slightly cloudier. President Trump vetoed the resolution, and the Senate could not override the veto.

A. War powers analysis

While a full analysis of the doctrine and history of the relation between Congress and presidents over war powers is beyond this discussion, the most widely-accepted framework for assessing questions of presidential power versus that of Congress comes from Justice Jackson’s famous concurrence in Youngstown Sheet & Tube Co. v. Sawyer. Jackson described three categories: 1) when the president acts pursuant to congressional authorization, he has maximum authority; 2) when Congress has neither authorized nor opposed action, there is a “zone of twilight” where the president can rely on his independent powers, but the scope is uncertain; and 3) “[w]hen the President takes measures incompatible with the expressed or implied will of Congress, his power is at its lowest ebb, for then he can rely only upon his own constitutional powers minus any constitutional powers of Congress over the matter.”

Here, Congress passed a resolution — but then the president vetoed it. Consequently, the resolution was not enacted into statutory law. But what is its implication for a constitutional test that focuses on the “expressed or implied will of Congress”?

The most straightforward interpretation is that, notwithstanding the president’s veto, Congress has expressed its will, and the passage of S.J. Res. 7 moved this situation squarely into the third Youngstown category.

Consider the opposite argument: that the vetoed S.J. Res. 7 has no effect, and Congress has entirely failed to express its opposition. That would require claiming that if a majority of both houses pass a resolution purporting to express Congress’s will, but at least one of the Houses falls short of two-thirds, then Congress has done nothing at all, and left the matter in the “zone of twilight” where it has neither authorized nor opposed action.

That would reverse the Constitution’s entrustment of the war power to Congress rather than the president. Virtually any president conducting an undeclared war would veto a resolution like S.J. Res. 7. Under a rule that Congress’s will is unclear when it passes a resolution ordering withdrawal but the president vetoes it, every presidential war would fall into the Youngstown zone of twilight unless Congress mustered two-thirds of each body. That would make hash of the War Powers Clause. As David French argues in National Review, “A declaration of war requires an affirmative act of Congress. A bipartisan majority’s rejection of American participation in the Yemeni conflict is anything but an affirmation.”

Kristen Eichensehr develops a middle position. She agrees that it’s absurd to call vetoed resolutions like S.J. Res. 7 “nullities.” Yet, she argues, in such cases “Congress hasn’t succeeded in perfecting its opposition to the President,” and the vetoed resolution “come[s] close to [Youngstown] Category 3, but not quite.” Instead, she suggests that Congress’s opposition justifies narrowly construing ambiguous presidential authorities within a Youngstown category 2 analysis. But while this interpretive canon may generate insights in other contexts, Eichensehr acknowledges that her approach is “a bigger lift” in the war powers context, where courts are almost never the final arbiters on the merits.

There’s something else odd about ascribing constitutional significance to Trump’s veto: the resolution was only even submitted to the president at all due to the procedural happenstance that the Senate chose to use a joint resolution rather than a concurrent resolution, which — for this purpose — would have been equally sufficient to express congressional will. (Black himself suggested a concurrent resolution as a step Congress should take before resorting to impeachment on this ground.) To be sure, the resolution’s Senate co-sponsors deliberately chose this vehicle, which has procedural advantages under the War Powers Resolution. But still: The Framers demanded the express affirmation of both houses of Congress for launching a war. When the president insists on prosecuting a war against the express opposition of both houses of Congress, should a momentous constitutional question turn on a point of parliamentary procedure?

Rather, as Justice Jackson explained, congressional will need not even be explicit — the issue is the “expressed or implied will of Congress.” If Congress’s will is clear, it doesn’t need to be expressed in an enacted law, let alone one backed by a veto-proof supermajority — certainly not in the war powers context, where the Constitution requires affirmative action by Congress.

B. Impeachment analysis

The unprecedented joint resolution dispatches three of Black’s four objections to impeachment over unauthorized wars. As for precedent, none of the previous examples of unauthorized military action involved defying a congressional joint resolution to end US involvement. As for clarity of congressional intent, barring a comprehensive statutory scheme (Black’s preference), the Cato Institute’s Gene Healy has noted that “Congress could draw a red line in an individual case.” So it did with S.J. Res. 7. And as for conflating the merits of the military action with the constitutional questions, the substantive deliberations about US involvement in Yemen have already been held by the very body that the Constitution entrusts with such decisions.

(Of course, these points apply only to President Trump’s continuation of US military intervention after Congress passed the Yemen resolution. Consequently, an article of impeachment against President Trump should only address continued prosecution of the war after May 2019.)

But Black’s preference for remedies short of impeachment deserves more discussion. Building on Black’s argument, Benjamin Wittes and Jane Chong have repeatedly argued that impeachment should be reserved for misconduct that Congress cannot stop by lesser means.

The obvious question is whether Congress must first override the veto of S.J. Res. 7 before considering impeachment. Certainly, the bill of particulars would be longer if the president were violating a statute as well as a constitutional provision. But requiring a veto override is contrary to constitutional structure. The Framers entrusted impeachment to the House by simple majority, but a veto override requires a two-thirds vote. Requiring a veto override as a prerequisite to impeachment would impose a new, non-textually-grounded supermajority requirement for impeachment.

Of course, Congress still has many lesser tools available. It could (still) pass a concurrent resolution. Or it could refuse to appropriate funds for the conflict. (The National Defense Authorization Act for 2020, passed by the House in July 2019, includes several provisions along those lines.)

Shouldn’t Congress exhaust other remedies first before resorting to impeachment? If impeachment’s only purpose were to stop the president from engaging in particular ongoing misconduct, that might be so.

But that’s not how the Framing generation thought about impeachment—certainly not in the case of unauthorized warfare. When Hastings was impeached, his military campaigns were already over; Blount was impeached for a military adventure that never even happened.

Here, impeachment serves other functions that wouldn’t be addressed by lesser remedies. Start with deterrence. Defunding one war doesn’t deter the same president, let alone others, from starting new wars. A president could rationally calculate that he can continue to start new wars and, at worst, Congress will compel him to withdraw. But that’s not much worse than the existing deterrent of an unpopular war weakening the president politically, as Vietnam did to Lyndon Johnson. In contrast, as John Adams confessed to Thomas Jefferson, a credible threat of impeachment can fill a president with “terror.”

As Black wrote, even for the specifically enumerated impeachable offenses of treason and bribery, we impeach a traitorous or corrupt president “principally because we fear he will do it again.” When a president persists in defying Congress and prosecuting a war over its stated objection, it’s reasonable to assume that he’ll continue to engage in undeclared wars, and Congress is not doomed to play whack-a-mole.

Finally, a critical purpose of impeachment is, in the words of Keith Whittington, “to articulate, establish, preserve and protect constitutional norms.” That seems to have been the purpose of Hastings’ impeachment. And the House impeachments of both Andrew Johnson and Justice Samuel Chase succeeded in establishing important constitutional norms, despite their ultimate acquittal. The norm here — that Congress, not the president, decides when to engage in warfare — has been fraying for years, but it is being undermined by President Trump even more than by Nixon, since Trump is continuing in defiance of explicit congressional disapproval.

In 1974, Edwards urged the Judiciary Committee to “send the message to future Presidents and this President about undeclared wars.” An article of impeachment would convey that message well.