Russian assets: how and why to seize them

I'm no fan of the Russian government here, but what exactly is the legal basis for seizing the Russian state's overseas assets?

It needs to be something much, much stronger than "well, they're obviously the Bad Guys", or, "UN says theyre bad", because while that might be true, no one invades a country thinking they are the baddies. The US invaded Iraq without UN sanction, for example, Turkey invaded Cyprus, the British of old invaded, well, everywhere, etc.

So it would seem to require some legal standard, and also some protection to stop legalistic fig leaves over a one sided majority out to get a country (eg imagine if the standard was "UN agrees invader is bad" and then Russia and China persuaded the UN that Japan had invaded Sakhalin Island because a fishing boat landed there.)

To repeat: Russia is the aggressor in Ukraine, but seizure of assets requires more than blog posts by "russiamustpay" dot com.

I need to read commentary on here more often so I don't feel so alone in thinking rationally. Thanks Kjell!