No belligerent can assume complete economic self-sufficiency. Britain needed imported food while exporting manufactures to earn the money that paid for those imports. War production also depended on overseas raw materials such as rubber and silk, with the latter used for parachutes. Even a continent-sized power such as the United States benefits from moving bulk goods by sea, with the Panama Canal connecting its coastal routes. Convoy protection therefore sustains both civilian consumption and the industrial inputs needed to fight.
Chapter · 6:49-9:36
Why wartime trade cannot simply stop
Open here for the economic case that even major powers depend on seaborne imports, exports, and coastal transport during war.