> The only way to really combat it is for different countries to collude and perhaps sanction tax havens.

Sounds like textbook prisoner's dilemma -- if they betray each-other then they both lose, but there's still a chance that they could betray their opponent without being betrayed themselves.

It seems to me that international law is riddled with subtleties where the general rule is "cheat effectively" rather than "don't cheat."