[Marine Inspector]

and it's a long-standing requirement of international law.

Wrong. It has nothing to do with international law.

Consular notification is an agreement between the United States and other countries. Some countries have mandatory consular notification and others are voluntary. Mexico is mandatory.

[BeHoldAPaleHorse]

Consular notification is an agreement between the United States and other countries.

Yes, the Vienna Convention on Consular Relations.

Some countries have mandatory consular notification and others are voluntary. Mexico is mandatory.

Because they are also a party to the VCCR.