[mseebach]

> there's nothing that says the host country can't put a captured foreign intelligence officer in prison

There is something that literally says that, which is article 29 of the Vienna Convention on Diplomatic Relations of 1961.

https://en.wikipedia.org/wiki/Vienna_Convention_on_Diplomati...

So, the way we as citizens are subject to the law of our respective governments, is the monopoly of violence. Obviously, a similar mechanism doesn't exist for international law[1], but violating a diplomat is technically casus belli.

Anyway, you're right in that the main sanction is the breakdown of diplomatic relations, but that doesn't mean that the ground rules are vague gentlemen's agreements and handshakes.

1: at least formally -- only a few countries would be able to get away with executing US diplomats without being treated to something resembling a monopoly on violence, yet the Vienna Convention is also respected among countries that could not do that