How Dualism May Save the United Kingdom from Brexit
Early in my international law education here in the U.S, I learned that dualism was an unfortunate concept that led to the U.S. violating international law obligations by failing to enforce those obligations (usually treaties) domestically. But today’s blockbuster decision from a UK court in Miller v. Secretary of State on Brexit should remind us that dualism can also work to protect international law. How? Well, if a country has many international obligations but is now seeking to withdraw from those obligations, dualism makes it harder to withdraw from those obligations.
In Miller, the court noted that although the UK Prime Minister usually has the unilateral authority to enter into and withdraw from treaties, that power cannot be used in anyway that would affect or change domestic UK law. Quoting an earlier decision, the High Court today noted that under the UK constitution, the Crown (through her ministers) has the sole and unreviewable power to make treaties. No Parliamentary assent or approval is needed. However,
[T]he Royal Prerogative, whilst it embraces the making of treaties, does not extend to altering the law or conferring rights upon individuals or depriving individuals of rights which they enjoy in domestic law without the intervention of Parliament. Treaties, as it is sometimes expressed, are not self-executing. Quite simply, a treaty is not part of English law unless and until it has been incorporated into law by legislation.
(Citing J.H. Rayner (Mincing Lane) Ltd. v Department of Trade and Industry  2 AC 418).
This basic principle seems to me crucial to the UK court’s holding today that the Crown (through her ministers) does not have the power to give notice under Article 50. Although the Crown would ordinarily have this power, the fact that triggering Article 50 would alter the domestic law of the UK makes this a question for Parliament.
In the US system, the President holds similar powers as the Crown and has similarly exercised unilateral powers to withdraw from treaties. But because treaties in the US have a vaguely monist character — they are self-executing and they have been approved by the Senate — it is harder to argue that the President cannot terminate treaties even if that termination would affect domestic US law. Why? Because if the treaty was “monist” and self-executing when made, then it is less troubling to unmake that treaty without going back to Congress. Unlike the UK, treaties are the supreme law of the land and directly preempt state law and earlier in time federal statutes. The kind of argument wielded by the Court in Miller just wouldn’t have any purchase here.
In any event, I don’t want to stretch this argument too far. The US may be facing its own Brexit moment soon if a President Trump makes good on his threat to withdraw the US from NAFTA. And if that happens (god forbid), expect pro-NAFTA folks to raise the case for congressional approval of any termination. But all in all, I think the dualist nature of the UK system aided the cause of the anti-Brexiteers in this case, which is a somewhat surprising result if you grew up learning that dualism was one of the great obstacles to a stronger international legal system.