During World War I, German U-boats operated solo except on one occasion. Initially, the British and nations supplying England with food and materiel scattered vessels singly across the ocean, making them vulnerable to the lone submarines. However, widespread late war re-adoption of the convoy system tipped the odds in the surface ships' favor, as one U-boat skipper described: "The oceans at once became bare and empty; for long periods at a time the U-boats, operating individually, would see nothing at all; and then suddenly up would loom a huge concourse of ships, thirty or fifty or more of them, surrounded by a strong escort of warships of all types."

World War I proved the value of submarines, ensuring their widespread employment in the next conflict, and one critical innovation in World War II's Atlantic U-boat operations consisted of wolf-pack tactics, in which Admiral Karl Dönitz put great faith: "The greater the number of U-boats that could be brought simultaneously into the attack, the more favorable would become the opportunities offered to each individual attacker.

Meanwhile, submarines exercised a decisive impact on the outcome of the Pacific Theater in World War II. The U.S. submarine fleet, largely though not exclusively under the overall command of Vice Admiral Charles Lockwood, strangled the supply lines and shipping traffic of the Empire of Japan. Their commerce raiding crippled both Japan's ability to keep its frontline units supplied and to manufacture the weapons, vessels, and vehicles needed to successfully carry on the struggle. Though constituting only 1.6% of the total U.S. Navy's tonnage in the Pacific, the submarine fleet inflicted massive losses on the Imperial Japanese Navy and Japan's crucial merchant marine. Submarines sank 55% of the merchant shipping lost, or approximately 1,300 vessels