Tonnage war

The British blockades of France were only possible so long as the Royal Navy retained the ability to defeat any French squadron venturing out from port.

Although the primary venue for the campaign was the North Atlantic, Hitler sent U-boats and surface raiders to all corners of the globe in search of the most efficient way to sink the maximum number of ships at minimum cost.

The U-boats campaign was very successful especially in the two "happy periods", of 1940 and of 1942, and was able to reduce the total shipping available to the Allies almost to a breaking point in 1943, when the tide of war started turning against Germany.

On the other side, an important factor in the British anti-submarine effort was the success of the codebreakers at Bletchley Park in reading U-boat messages encrypted using the German Enigma machine.

[3] The focus on U-boat successes, the so-called "aces" and their scores, the convoys attacked, and the ships sunk, serves to camouflage the Kriegsmarine's manifold failures.

To win this, the U-boat arm had to sink 300,000 GRT per month in order to overwhelm Britain's shipbuilding capacity and reduce its merchant marine strength.

These were "over-pessimistic threat assessments", Blair concludes: "At no time did the German U-boat force ever come close to winning the Battle of the Atlantic or bringing on the collapse of Great Britain".

There was another tonnage war in the Mediterranean, as Gibraltar convoys battled against Axis submarines and aircraft to deliver supplies and equipment to Malta, which was under siege by air and sea.

[11] During the summer of 1943, substantial numbers of American submarines were tasked with disrupting Japanese trade, in particular the cutting off of the flow of oil and other vital materials from the occupied territories of South-east Asia to Japan.

This too, became a tonnage war, with rapidly increasing results, and by mid to late 1944 Allied submarines and aircraft were experiencing difficulty in finding targets large enough to be worth a torpedo.

The Japanese merchant navy was all but wiped out, and despite desperate measures to make do without strategic materials, the war economy ground to a virtual standstill.

Compared to destroyers, merchantman were much more economical in fuel usage while having the capacity to carry full loads of troops plus sufficient equipment and supplies.

The Japanese Navy was in essence forced to "fight as uneconomical a campaign as could possibly be imagined", since in using destroyers they had to "expend much larger quantities of fuel than they wanted" considering Japan's disadvantage in oil supply, and this "fuel was used to place very valuable, and vulnerable, fleet destroyers in an exposed forward position while delivering an insufficient quantity of men and supplies to the American meatgrinder on the island".