At first, Russia’s argument about Crimea sounds a bit convincing: the best way to decide whether Crimea should remain part of Ukraine or instead secede and become part of Russia is by holding a referendum to let the people of Crimea decide.

On second thought, though, that creates major problems ahead of Sunday’s scheduled vote: what if the presence of Russian troops intimidates voters, so the process is not “free and fair”? What about the possibility that Russia is bussing in large numbers of native Russians to stack the deck?

Then there’s the bigger problem: the referendum seems inconsistent with the Ukrainian constitution, which says all Ukrainians would have to vote on Crimea’s secession – not just those living in Crimea.

These logical defects in the Russian proposal are obvious, and Secretary of State John Kerry no doubt brought them up during his meeting with Russian Foreign Minister Sergey Lavrov in London today.

But the real difficulty heading into this pivotal weekend for the crisis is more fundamental: the Russian proposal is based on an outdated theory of secession. Once upon a time, the right to secede was analyzed in terms of nationalist, linguistic, ethnic or religious homogeneity. Woodrow Wilson, for example, proposed redrawing the boundary lines in Europe to preserve the integrity of nationalist groups – Poland for the Polish, Serbia for the Serbians, and (now) Crimea for the Crimeans. This was thought to be the best way to promote self-determination and, therefore, democracy. If this is right, then all people living in Crimea should ideally vote to decide what to do. By this logic, the self-determination principle is the central consideration, and other problems – like intimidation – are just practical problems.

Does no one remember the former Yugoslavia? Using principles of self-determination to justify imposing ethnic homogeneity has resulted in genocide and ethnic cleansing. This brand of nationalism carried to its logical conclusion is ugly, plain and simple.

Arguments about ethnicity also overlook the central question: who owns the territory that constitutes Crimea? The answer is unambiguous: the Ukraine does. If people living in Crimea want to be Russian citizens, they can move to Russia – and that’s the right recourse. By voting for annexation to Russia, these would-be Russians are actually trying to take the territory away from Ukraine to give it to Russia. Their objective – and, of course, Russia’s, too – is not just to make these people Russian citizens but to take Ukrainian land, and it cannot be justified by a referendum about the preferences of those who live in Crimea today.

It’s a matter of international law: territory cannot be annexed simply because the people who happen to be living there today want to secede. If that were the case, then under international law, any geographically cohesive group could vote on independence. That would mean the Basques should be free from Spain and France, and the Kurds would have an independent nation; the large community of Cubans living in Miami could vote to separate from the United States.

If a referendum were the right way to decide these issues, Russia ought to be holding a referendum to determine the future of Chechnya. Of course, it isn’t.

International law is unambiguous on how countries should decide the fate of disputed territories like Crimea. Countries can acquire territory by discovering uninhabited land, signing a treaty – as with Khrushchev’s transfer of Crimea to Ukraine in 1954 – or occupying an area peacefully over a long period of time. The legal methods for resolving questions of sovereignty are founded on widely recognized principles of international law. These do not include, and have never included, a simple referendum of people living in a contested territory. That is why every successful secessionist movement has founded its claim on legal entitlement to the territory that they seek to “liberate”. Thus the Baltic states argued that they were illegally conquered by the Soviet Union; Tibet says the same about China; and Eritreans fought for decades to reverse their illegal annexation by Ethiopia.

What makes a secessionist claim successful in the eyes of the international community – indeed, in the eyes of the people fighting for secession – is the existence of a historical grievance over territory. No such legal claim can be made surrounding Crimea.

Make no mistake: what matters to Vladimir Putin about Crimea is not the thousands of Russian speakers living there, who allegedly might suffer at the hands of the new Ukrainian government. What matters is the land – the port and its value to the Russian fleet. No referendum is going to transfer valid title to that land to Moscow. Everyone knows that the territory in question is Ukrainian. At least, everyone should before Sunday.