When I took my first course in international law, I assumed there was no such thing. I thought it meant taking the League of Nations or the UN seriously.
I was, as everyone always does, taking it from the wrong end.
In the real world courts must decide on cases which involve more than one country. They have no choice. There is nothing theoretical about it. The courts in each country must pick its own precedents. American judges use a lot of British precendents and vice-versa, since both have a basis in the Common Law.
It had not occurred to me before I took the course that such cases HAVE to be decided, and the result is called international law, thought it is not international.
We have a similar problem with regard to interstate relations in the United States. The Constitution requires each state to give “full faith and credit” to the laws of other states. In other words, if one state allows homosexual marriage every other state must recognize it.
But they don’t. Since the beginning states have refused to extradict people convicted of criminal acts to the states where they were convicted.
There is the provision for full faith and credit, but there is no ENFORCEMENT mechanism. “Full faith and credit,” like international law, is a guideline, not a law.
If the Supreme Court told the president he had to use force to make one state extradict to another he would do it. Today the courts rule absolutely. But historically that was not the case.
When America extradicts a murderer from abroad, it has to guarantee the country extradicting that the nice guy will not face the death penalty here. Massachussetts can and probably will do exactly the same for murderere who escape there.
In Massachussetts the state supreme court decided that it didn’t like the state law that required that people cannot be married unless they are of different sexes. No other state accepts that.
In the United States you can be legally married to two different spouses. About 1945 North Carolina decided it would not accept Nevada’s easy divorce law. In fact until 1948 there was NO divorce law in SOUTH Carolina.
The court had no way to force North Carolina to accept Nevada divorces. So a man who was divorced and remarried in Nevada remained married to his original wife when he was in North Carolina but he was also legally married to his new wife in Nevada and states that accepted Nevada law.
As a matter of fact tens of thousands of people were and are legally married to two different partners in the United States. If any of them contracted a gay marriage in Massachussetts, they would legally married to three different people.
Under international law, America in general did not accept the easy Mexican divorces, even Nevada. So you could have your fourth spouse down there.
In case you think this is pure theory, remember that a person married to an American citizen has the right to permanent residence in the United States.
Here is a person married to an American citizen in half the states but not in the other half. If both wives are foreigners, which one gets the permanent visa?
Which gives you an idea of what international law looks like.