Legal sovereignty is a political concept that gives States the power to govern themselves and the people within a state. It is the ultimate source of legal power and autonomy. In a monarchy, a king or queen has sovereign power. It is sovereignty that allows a State to create laws, like a constitution, and create/protect its border from other sovereign States. That is, one State cannot interfere with another State’s sovereign power.
Legal sovereignty is an important concept in international law because it begs the question: If a State has supreme power to govern itself, who has the power to govern all states or some of them? More importantly, who has the power to prosecute those who interfere with another State’s legal sovereignty. The answer is no one. Compliance is voluntary. States can agree to governing principles in a treaty but there is no rule that says every State must sign. Further, when one State breaches the rules in a treaty, enforcement can only occur if the breaching State agrees to the enforcement process (i.e. investigation, prosecution, judgment) itself. If they don’t, there is no supranational body or supreme power that can force them to.
That is why many scholars say there is no such thing as international law. According to them, what we know as international law is simply a series of concepts and principles, that can be voluntarily complied with, but it has no “teeth” to force compliance.
An oft-cited example of this practical limitation is seen when the International Criminal Court (ICC) attempts to punish individuals for war crimes, crimes against humanity and/or genocide. The matter can only be investigated, prosecuted and punished with the cooperation of the State where the crimes occurred and the individual suspects. The ICC doesn’t have its own police force to execute arrest warrants and they don’t prosecute defendants who are not present in court. That means that an individual defendant could escape prosecution by just not showing up.