This has always been the case, and not just the US either. International law has no enforcement mechanism - it depends on willingness of countries to follow it or force others to. It does not have a proper system of courts to decide the law. Different bits of it can clash with each other. https://researchbriefings.files.parliament.uk/documents/CBP-...
International law has a number of enforcement mechanisms.
> it depends on willingness of countries to follow it or force others to.
All law depends on the willingness of its subjects to follow it, and failing that of its other subjects to force them to. This is not unique to international law.
Some systems of law (e.g., typical modern national criminal law) may have a particular group of people (usually with a formal heirarchy) who are expected to do the executive part of enforcement, and a similar (possibly the same or overlapping, but often distinct) group of people employed to do the adjudicative part of enforccement. International law has the latter (in several forms), but lacks the former. But anyone who is familiar with more than a narrow range of the most idealistic systems of national law will be aware that that that executive body can be a single point of failure—the real problem with international law isn't that it lacks such a single dedicated executive body, but that the important issues under it frequently involve significant conflicts of interest for any of the groups with the capacity to take on the executive role in the particular case, which is problematic under any system of law whether it has a single dedicated body for the executive part of enforcement or whether it relies on ad hoc case-by-case posses for that purpose.