One of the major lessons of World War II was the importance of coastal waters. It was not widely recognized beforehand just how vital the control of such waters would become, both in defending essential convoys as well as attacking those of the enemy, and in paving the way for amphibious landings.

While land based aircraft could carry out offshore operations by day and destroyers and cruisers patrolled deeper waters, the ideal craft for use in coastal waters were motor boats armed with torpedoes and light guns. But with the exception of Italy, none of the major powers had more than a handful of these boats operational at the outbreak of war.

From a small beginning, large fleets of highly maneuverable motor torpedo boats were built up, particularly by Britain, Germany and the USA. They operated mainly at night, because they were small enough to penetrate minefields and creep unseen to an enemy's coastline and fast enough to escape after firing their torpedoes. They fought in every major theater of war, but the first real threat came in the North Sea and English Channel from German E-boats, crossing to attack Britains vital convoys. Ranged against them in the battle of the little ships were British MTBs and MGBs and, later, American PT boats. They often fought hand to hand at closer quarters than any other kind of warship in a unique conflict that lasted right to the end of the war.

The E-boat Threat describes the development of these deadly little craft, the training of their crews who were usually volunteers and the gradual evolution of tactics in the light of wartime experience. Methods of defense are also related, which included the use of aircraft and destroyers as well as motor gunboats, sometimes acting under a unified command.