The United States Merchant Marine refers to either United States civilian mariners, or to U.S. civilian and federally owned merchant vessels. Both the civilian mariners and the merchant vessels are managed by a combination of the government and private sectors, and engage in commerce or transportation of goods and services in and out of the navigable waters of the United States. The Merchant Marine primarily transports cargo and passengers during peacetime; in times of war, the Merchant Marine can be an auxiliary to the United States Navy, and can be called upon to deliver military personnel and materiel for the military. Merchant Marine officers may also be commissioned as military officers by the Department of Defense. This is commonly achieved by commissioning unlimited tonnage Merchant Marine officers as Strategic Sealift Officers in the United States Navy Reserve.