Significant public discussion has focused on whether airstrikes against ISIL in Syria violate the international legal prohibition on the use of force. Remarkably what has largely escaped attention is that the very same question applies to the ongoing arming and training of the Syrian rebels. Under international law, arming or training rebels also constitutes a “use of force”—and can only be justified by the consent of the territorial state,  the right of national or collective self-defense, or Security Council authorization. Let’s put it this way: if the U.S. theory of collective self-defense is legally valid (or invalid) in justifying airstrikes in Syria, so would it presumably be valid (or invalid) for arming and training Syrian rebels.

That basic point has several legal and policy implications. Consider a few examples. First, France has acknowledged that it has armed rebel forces inside Syria, yet the French President recently expressed uncertainty about whether France has the legal right to use force in the country without Syria’s consent (by joining U.S.-led airstrikes). President Holland might be told that France has essentially crossed that Rubicon long ago. Second, some questions of U.S. domestic law may turn on whether the President has the authority to violate the UN Charter without specific congressional authorization. That analysis should be informed by the fact that the Congress has already voted—through legislation to train and equip rebel forces—to use force inside Syria without the Assad government’s consent. Third, as Congress considers draft legislation authorizing force against ISIL, some congressional members will question whether the U.S. has the right under international law to use force against ISIL in Syria. They should know that Congress has already voted to take that step.

We note that under the most widely accepted view of international law on the use of force, airstrikes are generally considered a greater intrusion on a state than arming and training rebels. According to the framework set forth by the International Court of Justice in Nicaragua v. United States (1986), airstrikes can constitute an “armed attack” triggering the target state’s right to self-defense; but the arming and training of rebels does not on its own amount to an armed attack. [As an aside: the long-standing United States’ position on these matters, in contrast, is that any use of force (below the threshold of an armed attack) can also trigger the right of self-defense.]

In two respects, however, current airstrikes in Syria may actually be more consistent with international law on the use of force than U.S. efforts to arm and train Syrian rebels.  First, exercise of the right of self-defense in Syria must be balanced against that State’s sovereignty.  Somewhat paradoxically, surgical and limited airstrikes against ISIS are arguably a lesser violation of Syria’s sovereignty than arming and training rebels to fight ISIS when those rebels will also use the arms and training in operations against the government.  Second, current U.S.-led airstrikes are justified on the basis of the right of collective self-defense to quell the ISIL threat to Iraq. But the collective self-defense of Iraq is not the sole justification and purpose of arming and training rebel forces. As one of us (Mike Schmitt) has written (and both of us agree), a decision to arm and train Syrian rebels to fight Assad in 2013 would have violated the prohibition on the use of force. Mike wrote:

“There is, therefore, little doubt that the provision of lethal aid directly to the Syrian rebel forces would amount to a ‘use of force,’ at least by the generally accepted standards the ICJ [International Court of Justice] set forth in Nicaragua. Absent a justification for such use, the provision of lethal aid would violate the U.N. Charter’s prohibition on the use of force, as well as the analogous prohibition resident in customary international law.”

[See also this post by Ashley Deeks over at Lawfare in May 2013].

The widely reported “covert” U.S. program to train Syrian rebels and the recent congressional legislation may run afoul of international law because they are not predicated on a right of self-defense (or any other exception to the prohibition on the use of force). Neither of those U.S. policies appears to be focused exclusively on fighting ISIL. The former reportedly occurred well before the rise of ISIL inside Iraq. And the recent congressional legislation for Syrian rebels provides funds for multiple purposes including “securing territory controlled by the Syrian opposition” and “promoting the conditions for a negotiated settlement to end the conflict in Syria.”

One implication of this analysis is that Congress may have already authorized action outside the use of force regime, even if U.S. air operations are consistent with that regime. A second implication is that the focus on U.S. airstrikes has obscured another set of actions carried out by states inside Syria, which may disrupt the international legal system on the use of force in far-reaching ways. None of this is to say that arming and training the rebels to undermine the Assad regime is illegitimate or unsound. It is a reasonable positon to take that such action is unlawful but legitimate and morally required. Regardless of those policy questions, thorough reporting and analysis of the legality of use of force that arise in the case of ongoing military operations in Syria should appreciate these other use of force questions lurking in the background.

The views expressed in this piece are those of the authors in their personal capacity.