The phrase "national borders" refers to the boundaries or lines that separate one country from another. It's like an imaginary line that determines the limits of a particular nation's territory.
To protect trade and trafficking of endangered animals, it is almost impossible to get permission to transport an endangered animal across national borders even for captive breeding programs. (sciencedaily.com)
Researchers, to the extent that their work required them to collaborate with colleagues beyond national borders, had to scale high boundaries to do so. (scientificamerican.com)
Hence, those who turn to economists for guidance in policy formation, are encouraged to reduce government interference in markets and the role of national borders in restricting the market. (religion-online.org)