The Secretary of Transportation shall arrange for the employment of the Department of Transportation’s vessels in steamship lines on such trade routes, exclusively serving the foreign trade of the United States, as the Secretary determines are essential for the development and maintenance of the commerce of the United States and the national defense. However, the Secretary shall first determine that those routes are not being adequately served by existing steamship lines privately owned and operated by citizens of the United States and documented under the laws of the United States.