The convoy system, which can be defined as a group of merchant vessels sailing together, with or without naval escort, for mutual security and protection, has a much longer history than sometimes suggested. It was commonly employed during the Age of Sail, notably by British vessels under threat from French and US commerce raiders, and indeed probably has its origins in ancient times.
However on the outbreak of World War I in 1914, it was widely argued that the advent of steam had rendered the convoy obsolete. It was felt that the necessary delays involved in assembling vessels into convoy would take up too much time, as would the limitation of a convoy's speed to that of the slowest ship. Many captains argued that such a concentration of shipping would bring with it an unacceptable risk of collision, as well as presenting an easy target to submarine attack. The British Royal Navy felt that escorting convoys was too "defensive" a use of their ships, continuing to advocate independent hunting groups, despite the disappointing results these soon proved to have.
Although convoys were introduced in the early part of the war for ships on the short cross-English Channel routes, partly because it was generally assumed that naval escort on a one-to-one ship base was necessary, it was assumed that trans-Atlantic protection would be impossible.
It took the looming U-boat crisis following the entry of the USA into the war in April 1917 to bring about a re-think. With sinkings of merchant ships reaching a level which threatened to force Britain out of the war within months, the Admiralty looked again at the statistics. It was found that most losses fell on ocean-going trade, less than 10% of total shipping, a sufficiently small number to make convoy escorts feasible. The result was a gradual introduction of a convoy system and the construction of growing numbers of dedicated escort vessels to supplement fleet destroyers. Where the convoy was introduced, sinkings were reduced to 1.1% of their previous figure. In the nick of time, the U-boat threat had been countered.
