> Is it not a three-stage process? I found that treaties seem to be unenforceable unless there is a legally mandatory statutory implementation of them, regardless of whether they are ratified.

As far as international law is concerned, once a treaty has been ratified and entered into force, it is binding on the ratifying state, they have an international legal obligation to obey it, and they can be subject to consequences under international law if they violate it. The nature of those consequences vary greatly depending on the details of the treaty - often treaties have dispute resolution mechanisms to be invoked if one party claims another is violating it (such as the ICJ, arbitration, WTO dispute settlement, etc). Trade treaties often permit imposition of tariffs in cases of violation. In extreme cases, violating a treaty could even result in military action (e.g. what happens if you sign a peace treaty to end a war and then decide not to comply with it?) On the other hand, many treaties are rather toothless in that they fail to provide any real consequences for violations. Still, just because there might not be any real consequence for the violation, doesn’t negate the violation’s legal existence.

Coming to your question about implementing legislation - different countries have different systems. In countries with a “monist” system, international law is considered part of domestic law, and so a treaty once ratified automatically becomes part of the law of the land. Conversely, in countries with a “dualist” system, international law and domestic law are viewed as two independent spheres, and the domestic legal system will not consider a treaty binding absent domestic implementing legislation. And “monist” and “dualist” are ideal types, and some national legal systems are actually hybrids that don’t neatly fit in either category, combining elements of both - they may be monist with respect to certain categories of treaties and dualist with respect to others. Even in a purely monist system, some treaties might be considered “non-self executing” - for example, some treaties require states to criminalise certain acts under their domestic law, but leave the detailed definition of those crimes up to each state party - even in a monist system, such a treaty will likely be viewed as domestically ineffective absent domestic implementing legislation, since the crimes it seeks to create are too vaguely defined to actually be prosecuted. Also, in some countries with a federal system, e.g. the US, implementing legislation may be required at both the federal and state levels; in others, the federal level has the power to impose treaties on the states, even in areas where it would not normally have legislative competence (e.g. the external affairs power under the Australian constitution)

However, from an international law perspective, the question of whether domestic implementing legislation exists is irrelevant. If a state ratifies a treaty and it enters into force, they have an international legal obligation to obey it - and if they fail to do so because they haven’t enacted the necessary domestic legislation, international law does not consider that a valid excuse-they are guilty of violating it, and have to face the consequences of that violation, whatever those may be.

You are talking about the Vienna Convention on Consular Relations. There was a 2001 ICJ case on that, the LaGrand case (Germany vs United States). The LaGrand brothers were born in Germany, moved to the United States, never became US citizens, stabbed a man to death in 1982 in Arizona in the course of a bungled bank robbery. Under the Convention, they had the right to German consular assistance, and US authorities had the duty to inform them of that right, but failed to do so. They were sentenced to death, and the state of Arizona executed them in 1999. Germany got an order from the ICJ that the execution not go ahead, but the US chose to defy the ICJ order and execute them anyway. In 2001, the ICJ found that the US had violated Germany’s rights under the Convention by so doing. However, given Germany did not make any request for damages, and the US decided to formally apologise to Germany, the ICJ did not impose any penalties on the US for the violation.

The US argued that under the US constitution the federal government was powerless to compel the state of Arizona to comply with the Convention or the ICJ’s order. Germany even filed a case with the US Supreme Court seeking it to compel Arizona to comply, but it ruled that under the US constitution Arizona didn’t have to. However, international law doesn’t care about the US constitution. If the US constitution prevents the US from obeying international law, that’s an internal US problem of zero relevance to other countries or to international institutions such as the ICJ. Given there are over 190 sovereign states in the world, if national constitutions were an excuse for disobeying international law, international law would quickly turn into a dead letter. Also, while US law considers the federal government and state governments to be “separate sovereigns”, as far as international law is concerned, it is all one country, and the US (represented by the President and State Department) is internationally responsible for the acts of all its subdivisions, and if the federal government lacks the constitutional authority to make states obey international law, that’s its internal problem, with which other countries ought not need to be concerned