When does an independence movement get to form its own nation? For decades, a set of unstated but well-known rules has supposedly decided that.

But those rules include a number of contradictions. And as the Catalans in Spain and the Kurds in Iraq push for independence through referendum, those contradictions are becoming uncomfortably clear.

The first rule: Portray your cause as a struggle for democracy and human rights first, national self-determination second. This helps get around the fact that there is no legal right, under international or often domestic law, for secession.

Also, get the great powers, or at least the United States, to compel, coerce or bribe the government you’re trying to break from into going along. If the breakup isn’t mutual, it’s almost impossible to peacefully resolve, so better to pretend everyone wants it.