The Defense Department’s reliance on a broad interpretation of “collective self-defense” appears to authorize military operations further removed from congressional approval or other legal limits. Earlier this month Senator Tim Kaine (D-Va.) wrote to Secretary Mattis expressing alarm “that the Department of Defense believes that, by merely designating a group as a partner force, it can respond with military action to protect that partner force and its property if threatened by any group—even one that poses no direct threat to the United States, its Armed Forces or persons, nor is covered by an AUMF.” Senator Kaine, who has long expressed the need for Congress to reassert control over war powers and is far from alarmist, called the Defense Department’s legal interpretation “broad and troubling” and cautioned that the “unintended consequences of this policy could be grave.”

But just how broad is the authority the Defense Department is claiming, and is it even new?

The legal interpretation that has Senator Kaine so concerned came to light as a result of an ongoing exchange between the Senator and the Defense Department about Niger. It began when the Senator started digging deeper into the Defense Department’s legal justification for the operation in western Niger on October 4, 2017 that resulted in the deaths of four American soldiers. In a May 23, 2018 letter that was part of this ongoing exchange, Senator Kaine asked Secretary Mattis specifically whether the theory of collective self-defense could be invoked “to support foreign partner forces engaged in hostilities against enemies that are not covered by a congressional authorization for use of military force?”

According to Kaine’s latest letter, the Defense Department provided a detailed response to the May 23 letter, but only select portions were unclassified. In those unclassified portions, which Senator Kaine quoted in his October 2, 2018 letter, the Defense Department explained that “collective self-defense” in this context referred to an “extension of unit self-defense to foreign or irregular partner forces or persons” and that when collective self-defense is permitted, as a supplemental rule of engagement, “U.S. forces are permitted to defend partner forces from attack or an imminent threat of attack with necessary and appropriate force.” Such responses to an attack are not limited to particular groups or individuals, “including not being limited to groups covered by the 2001 AUMF or other congressional authorizations for the use of force.”

The Defense Department’s explanation then refers to U.S. forces using force on February 7, 2018 against Syrian pro-regime forces who had attacked a Syrian Democratic Forces base and to the October 4, 2017 Niger incident as examples of collective self-defense.

Is the collective self-defense legal theory the Department provided to Senator Kaine new? Readers will recall that in March the Trump administration submitted a “Report on the Legal and Policy Frameworks Guiding the United States’ Use of Military Force and Related National Security Operations” that was required by Congress in the FY 2018 National Defense Authorization Act. In the unclassified sections of that report, the administration provided an explanation for these same two incidents, as well as at least three others, based on defense of partner forces. For example, with respect to Syria, the report explains:

In May and June 2017, as well as February 2018, the United States took strikes against the Syrian Government and pro-Syrian Government forces. These strikes were limited and lawful measures taken to counter immediate threats to U.S. or partner forces while engaged in the campaign against ISIS. As a matter of domestic law, the 2001 AUMF provides authority to use force to defend U.S., Coalition, and partner forces engaged in the campaign to defeat ISIS to the extent such use of force is a necessary and appropriate measure in support of counter-ISIS operations. As a matter of international law, necessary and proportionate use of force in national and collective self-defense against ISIS in Syria includes measures to defend U.S., Coalition, and U.S.-supported partner forces while engaged in the campaign to defeat ISIS.

Similarly, in the section on Niger, the report explains:

At the request of the Government of Niger, the previous Administration approved, and the current Administration continued, the deployment of U.S. forces to Niger under the President’s constitutional authority as Commander-in-Chief and Chief Executive and under certain statutory authorities of the Secretary of Defense to train, advise, and assist Nigerien partner forces. On October 4, 2017 and December 6, 2017, those U.S. forces and their Nigerien partner forces were attacked by forces assessed to be elements of ISIS, a group within the scope of the 2001 AUMF, and responded with force in self-defense. The Administration has concluded that this use of force was also conducted pursuant to the 2001 AUMF.

The administration was clearly invoking the right to defend partner forces with these explanations back in March, but in each of these examples the administration also carefully tethered the authority to domestic statutory authorizations. With respect to Syria, they claimed the authority to defend partner forces was granted by the 2001 AUMF to the extent that it was a necessary and appropriate measure in support of the fight against ISIS (setting aside the debate over whether that fight itself is even covered by the 2001 AUMF). And with respect to Niger, while the administration also relies on Article II authorities, the report additionally cites statutory train, advise, assist, and accompany authorities.

While the tethering of collective self-defense to a statutorily authorized reason for U.S. forces to be present in the first place is an important constraint, these justifications would still provide extraordinarily expansive powers to stretch the 2001 AUMF, as well as train, advise, assist, and accompany authorities, in ways that Congress never intended. In passing the 2001 AUMF, Congress did not intend to give the president, nor the Pentagon, the authority to mount a multi-country campaign against an enemy not affiliated with the original 9/11 attack, let alone empower the U.S. military to protect non-state partner forces against an unrelated nation-state’s irregular military forces. Additionally, in indefinitely authorizing the train, advise, assist, and accompany authorities, Congress did not seek to authorize U.S. participation in another country’s hostilities against a group not recognized under an existing statutory authority or without prior Congressional oversight.

Even such statutorily tethered authorities could trigger the grave policy consequences that worry Senator Kaine. For example, such collective self-defense authority could enable the executive branch to circumvent Congress’ role in authorizing the use of force, would risk entangling the United States in other countries’ conflicts, and risk triggering a conflict between the United States and other nations. In addition, this expanded interpretation exposes the folly of passing overly broad authorizations for the use of military force that delegate authority to the executive branch to make decisions that should be under the purview of Congress. In a similar threatened power shift, the Senate’s latest AUMF proposal would have handed over Congress’ authority to determine who, when, and where the United States goes to war. By failing to include geographic limitations, a sunset provision, or name a specific enemy in the 2001 AUMF, Congress would have enabled the executive branch to stretch this AUMF far beyond recognition and potentially use it as a justification to use military force around the world in contexts never conceived by Congress with limited congressional oversight (if any at all). Fortunately, the Senate abandoned this effort but still has not found a path to reining in existing AUMFs and the executive’s continued stretching of them.

Yet there is reason to be concerned that the Defense Department’s legal theory goes far beyond troubling stretching of statutory authorities. The portions of the Defense Department’s explanation quoted in Senator Kaine’s letter, and his level of alarm, suggest that the Defense Department may be going far beyond these statutorily-tethered claims of collective self-defense articulated in the March 2018 framework report. It appears that the Defense Department could be making a new legal interpretation that would claim that it has the authority to launch U.S. military involvement on behalf of a partner force just because the executive branch has decided to do so. According to Senator Kaine’s follow up letter, the Defense Department did not offer a definition of a partner force outside a group designated as such by the president. This would be an astounding expansion of the executive branch’s interpretation of collective self-defense and the president’s claimed Article II power.

If that’s the case, the Defense Department should not only respond to the list of questions in Senator Kaine’s October 2 letter, but must also report this legal interpretation in unclassified form to the relevant Congressional committees. Under the same provision that required the March 2018 framework report, the President is required to notify the relevant congressional committees within 30 days of any change to the legal and policy frameworks for the United States’ use of military force and related national security operations. This report must also include the legal, factual, and policy justification for the change.

Senator Kaine’s letter suggests that the Defense Department may be claiming the authority to use military force anywhere in the world to defend any entity—state or nonstate—that it designates a “partner force.” As such, Congress should require the Defense Department’s General Counsel to testify in a public hearing to publicly clarify what the Defense Department claims its collective self-defense authority to be and provide an accounting all groups deemed U.S. partner forces around the world. These explanations are critical to understand just how broad of an authority the Defense Department is claiming. As the senator’s letter implores, what are the limiting principles of such a sweeping authority?

Photo of U.S. Marines stationed at Camp Lemonier, Djibouti in 2003 by Sean Gallup/Getty Images.