A submarine cannot aim at a ship without some object as a sight.  So one submarine often acted as a “sight” for the submarine firing the torpedo.  Submarines, which at first were unarmed, were later fitted with armour plate and cannon were mounted on deck.  The biggest submarines now carry 6-inch guns.

Like all methods of ruthless warfare the submarine campaign can be and will be for a time successful.  Germany’s submarine warfare today is much more successful than the average person realises.  By December, 1916, for instance, the submarines were sinking a half million tons of ships a month.  In January, 1917, over 600,000 tons were destroyed.  On February nearly 800,000 tons were lost.  The destruction of ships means a corresponding destruction of cargoes, of many hundreds of thousands of tons.  When Germany decided the latter part of January to begin a ruthless campaign German authorities calculated they could sink an average of 600,000 tons per month and that in nine months nearly 6,000,000 tons of shipping could be sent to the bottom of the ocean,—­then the Allies would be robbed of the millions of tons of goods which these ships could carry.

In any military campaign one of the biggest problems is the transportation of troops and supplies.  Germany during this war has had to depend upon her railroads; the Allies have depended upon ships.  Germany looked at her own military situation and saw that if the Allies could destroy as many railroad cars as Germany expected to sink ships, Germany would be broken up and unable to continue the war.  Germany believed ships were to the Allies what railroad carriages are to Germany.

The General Staff looked at the situation from other angles.  During the winter there was a tremendous coal shortage in France and Italy.  There had been coal riots in Paris and Rome.  The Italian Government was so in need of coal that it had to confiscate even private supplies.  The Grand Hotel in Rome, for instance, had to give up 300 tons which it had in its coal bins.  In 1915 France had been importing 2,000,000 tons of coal a month across the Channel from England.  Because of the ordinary loss of tonnage the French coal imports dropped 400,000 tons per month.  Germany calculated that if she could decrease England’s coal exports 400,000 tons a month by an ordinary submarine campaign that she could double it by a ruthless campaign.

Germany was looking forward to the Allied offensive which was expected this Spring.  Germany knew that the Allies would need troops and ammunition.  She knew that to manufacture ammunition and war supplies coal was needed.  Germany calculated that if the coal importations to France could be cut down a million tons a month France would not be able to manufacture the necessary ammunition for an offensive lasting several months.