Whatever verbal agreements President Donald Trump and President Vladimir Putin may have reached in their private meeting in Helsinki, a congressional limitation on cooperation with Russia may put a stop to some of those efforts. Several years ago, Congress specifically placed roadblocks in advance of any future military cooperation with Russia. By a federal statute, consistently renewed on an annual basis, Congress prohibits bilateral military cooperation with Russia—with some “buts” added to that legal prohibition. (Disclaimer: I worked on issues involving this statutory provision when I served as special counsel to the General Counsel of the Department of Defense.)

How exactly this legal prohibition by Congress works—and whether the administration has a constitutional basis for ignoring it—has become ever more pressing following the “read out” of the Helsinki Summit and remarks by the head of U.S. Central Command, Gen. Joseph Votel, who invoked the statutory prohibition explicitly on Thursday.

During the two-hour private meeting in Helsinki, President Putin made “specific and interesting proposals to Washington” on how the two countries could cooperate on Syria, and the two leaders entered into “important verbal agreements.” That’s according to what Russia’s Ambassador to the United States, Anatoly Antono, told reporters. Meanwhile, “officials at the most senior levels across the U.S. military, scrambling since Monday to determine what Trump may have agreed to on national security issues in Helsinki, had little to no information,” according to the Washington Post.

Some of those Pentagon officials may feel a sense of security that the congressional statute forecloses certain options. But does it? In response to a well-crafted question by NPR’s Tom Bowman, Gen. Votel told reporters on Thursday: “The National Defense Authorization Act, as a law, prohibits us from coordinating, synchronizing, collaborating with Russian forces.”

There is a common misunderstanding of the statute made by commentators—and perhaps by some policymakers in the current administration. It is commonly thought that the federal law categorically prohibits “bilateral military-to-military cooperation between the Governments of the United States and the Russian Federation.” After all that’s what the statute says. But the statute also says that the Secretary of Defense, in coordination with the Secretary of State, can break out of this constraint if the Defense Secretary determines a “waiver is in the national security interest of the United States.” Such a decision by Defense Secretary Jim Mattis could not be kept secret. The statute requires him to notify congressional committees of any such waiver. In essence, Sec. Mattis is what one might call a “veto player”—he is formally assigned the authority to make the determination. Not the President. The Defense Secretary. (But that fact raises a potential constitutional problem with the statute, which I outline below.)

The White House may try to find wiggle room in the wording of the statute or else pull out the big legal guns—a claim that the congressional conditions are unconstitutional and thus do not need to be followed. What’s the wiggle room? The statute is written to prohibit only “bilateral” military cooperation. So, what if a Trump-Putin arrangement involves multilateral cooperation, say, including other countries in the region like Turkey? It would be a problem for the administration to set up such a multilateral arrangement as a pretext for what all intents and purposes involves bilateral cooperation with Russian military forces. But that’s not much reassurance for those in the Pentagon who worry that the President may be headed down a path that does not serve the interests of the United States and that seeks to sideline Mr. Mattis in the process. On the Hill, members of Congress may want to amend the current version of the NDAA bill that extends this statutory prohibition so that the final legislation closes this potential “loophole.” If the military knows such a legislative fix is potentially in the works, that alone might provide sufficient cause for the administration not to start any “multilateral” operations involving Russia for which funding will likely be cutoff in the future.

Finally, an assessment of this law would not be complete without delving into some constitutional questions. The Trump administration might try to argue that it cannot be constitutionally constrained by the existing statute. The administration could try to pursue two lines of argument and claim one or both of the following:

Argument 1: Congress does not have the constitutional authority to assign this determination to the Secretary of Defense. On this view, the President could simply direct the Defense Secretary to issue the waiver or President Trump could decide the matter himself  on the ground that Congress cannot, as constitutional matter, dictate the inner workings of the Executive Branch.

Argument 2: Congress does not have the constitutional authority to impose certain conditions on bilateral military cooperation such as the substantive determinations that must be made to issue a waiver or the substantive conditions that prohibit military cooperation in the first place (the statute requires the end of Russian occupation in Ukraine, as one example, as a precondition for any future military cooperation).

There are naturally constitutional arguments in the opposite direction, especially since the statute involves a decision by Congress on the appropriation of funds. Arguments in favor of the statute include that Congress gets to decide if it does not want to fund some particular activity, and that Congress need not defer to the President’s judgment to permit military-to-military cooperation with a foreign adversary.

The legal issues that arise here are not specific to this statute. The same questions may have implications for other legislation that Congress considers in response to the President’s actions coming out of Helsinki.

For an answer to my questions, see former acting DOD General Counsel Robert E. Talyor’s piece here. Just Security will have even more discussion of these legal questions soon.