As co-blogger David Bernstein points out, the recent horrifying terrorist attack against France gives the Obama administration an opportunity to legalize its previously unconstitutional war against ISIS. Up until now, the war has been illegal because the president lacks congressional authorization for it. Assuming (as is highly likely), that ISIS was indeed behind the attacks, the United States has a legal obligation to help defend France under Article 5 of the 1949 North Atlantic Treaty, which created the NATO alliance. Here is the relevant text:

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 recognised 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.

France is a signatory of the North Atlantic treaty, and Friday’s brutal terrorist attack surely qualifies as an “armed attack against [France] in Europe.” Therefore, the US now has a legal obligation to consider the attack against France as an attack against the US itself, and to “assist” France with “such action as it deems necessary,” including “the use of armed force.”

Some scholars and jurists contend that it isn’t possible for a nation-state to be in a true state of war with a private terrorist organization. Even if this is correct, by this point ISIS qualifies as a state-like entity, having seized control of large portions of Syria and Iraq. In any event, the NATO treaty applies to all “armed attacks” on the signatories in “in Europe or North America,” regardless of whether they qualify as a war or not. The NATO allies did in fact invoke Article 5 after the 9/11 attacks against the US, an incident with obvious similarities to the attack on France.

Article VI of the Constitution makes treaties the “supreme law of the land” if they were made “under the authority of the United States.” This, at the very least, allows the president, with Senate ratification, to make legally binding commitments to use the powers of the federal government, which clearly include the power to wage war, and otherwise use military force. Article 5 of the North Atlantic Treaty is such a commitment, and it pretty clearly applies in this case.

The Alliance could formally invoke Article 5 by doing so at a meeting organized in accordance with Article 4, which requires member states to “consult together whenever, in the opinion of any of them, the territorial integrity, political independence or security of any of the Parties is threatened.” However, the US likely has a legal obligation to aid France under Article 5 even in the absence of an Article 4 consultation.

Article 5 provides a much stronger justification for the war against ISIS than the previous extremely dubious rationalizations presented by the Obama administration. But it cannot retroactively legalize the President’s previous illegal actions, or the similarly unconstitutional war against Libya in 2011.

Citing Article 5 might not be an attractive option for the Obama administration, because it might be seen as an implicit admission that the war was previously illegal (as it in fact was). For this reason, they might well decide not to rely on it. But it is nonetheless the only sound legal justification for continuing the war against ISIS, unless and until the president gets a new authorization from Congress.

As I have previously explained, the president’s failure to get congressional authorization for the wars against ISIS and Libya is not just a purely legal problem. In addition to creating terrible precedents for the future, it also has made it more difficult to build the kind of political consensus needed to wage war effectively. Invoking our Article 5 obligations to a key NATO ally may not fully cure these problems. But it would at least be a valuable step in the right direction.

UPDATE: James Stavridis presents a related argument for invoking Article 5 in this recent article in Foreign Policy. GOP presidential candidates John Kasich and Marco Rubio have also urged NATO action under Article 5. None of them, however, has noted the implications for the domestic constitutional rationale for the war.

UPDATE #2: Obviously, it might be politically difficult to rely on Article 5 if France does not want NATO assistance for whatever reason. However, President Francois Hollande has already called Friday’s attack an “act of war” pinned the responsibility on ISIS, and stated his determination to wage the war aggressively. Most likely, therefore, the French would welcome Article 5 support from the US and other NATO allies.

UPDATE: International law scholar Julian Ku has put up a a critique of this post, arguing that Article 5 is not enough to justify US intervention against ISIS without additional congressional authorization:

I agree with Ilya that the Obama Administration’s current domestic legal justification for the war against the Islamic State is sketchy at best. But I am not sure I agree with him that Article V should be read as a “pre-authorization” for the President to use military force without going back to Congress for a specific authorization…..

I agree that the horrible Paris attacks would constitute an “armed attack” on a member of NATO “in Europe or North America.” But I don’t think Article V requires the other NATO members to provide military assistance. Rather, “if such an armed attack occurs,” a NATO member “will assist the Party so attacked [France]…by taking forthwith…such action as it deems necessary, including the use of armed force.” (emphasis added).

I read this language as requiring the U.S (for instance) to assist the attacked party (France), and that this assistance could “include the use of armed force.” But I don’t think it has to.

Moreover, Article IX of the North Atlantic Treaty states that “[t]his Treaty shall be ratified and its provisions carried out by the Parties in accordance with their respective constitutional processes.” (emphasis added). I read this as requiring Parties to carry out provisions like Article V “in accordance with their respective constitutional processes.” If you are someone who believes that Congress must authorize the use of force by the President in most cases, than this language would mean that the President has to go back to Congress.

Julian makes a reasonable point. But I remain unconvinced. While the use of force is discretionary under Article 5, treating an attack on an ally within the designated area as if it were an attack on the US itself is not: “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 in the event of an enemy attack on the US itself, the president has the legal authority to use force of his own volition, without additional congressional authorization. While the president may choose not to use force (and Congress may choose to forbid him to do so), the proper invocation of Article 5 does give him the same authority to use force as he would have in the event of an attack on the United States itself. All of this is in accordance with US “constitutional processes,” just as Article 11 of the North Atlantic Treaty requires. By contrast, when Obama attacked Libya in 2011 and when he began the war against ISIS last year, there was no comparable treaty obligation.

Julian also claims that “[t]he main legal purpose of Article V was (is) to allow NATO countries to act consistently with the U.N. Charter’s limitations on the use of force (such as they are).” I disagree. The true main purpose of Article 5 is to commit the signatories to a system of collective defense against attack – a commitment originally necessitated by the threat of the Soviet Union, but not limited to that specific danger. Empowering the president to assist an ally under attack without having to seek congressional authorization pretty obviously facilitates that purpose, as it makes the US commitment to defend its European allies more credible and certain.

UPDATE #3: Christopher Preble makes an argument similar to Julian Ku’s here. I think my response to Ku applies to him, as well. Preble also notes that NATO did not invoke Article 5 after terrorist attacks on Spain and Britain in 2004-05. But those attacks were launched by terrorist groups linked to al Qaeda that the NATO powers were already at war with, thanks to the invocation of Article 5 after the 9/11 attacks in 2001.