What is the primary condition for a vessel to enjoy the right of innocent passage through a foreign territorial sea?

The primary condition for a vessel to enjoy the right of innocent passage through a foreign territorial sea is that the vessel must be engaged in innocent passage. Innocent passage refers to the right of ships to pass through the territorial waters of a coastal state, provided that such passage is not prejudicial to the peace, good order, or security of the state. This principle is enshrined in international law, particularly the United Nations Convention on the Law of the Sea (UNCLOS).

For passage to be considered innocent, it generally must serve the purpose of traversing the waters without any intention of threatening the coastal state's security or engaging in harmful activities, such as fishing, conducting research, or any form of military exercise that may arise. Thus, as long as the vessel adheres to these conditions and navigates in a manner that does not violate the rules established by the coastal state, it enjoys the right to innocent passage.

In contrast, being registered under a flag state, having military designation, or abstaining from anchoring are not universal requirements for innocent passage. While a vessel must be flagged to be considered lawful under international law, the crux of innocent passage lies more fundamentally in the character of the passage itself rather than the specific attributes of the vessel.