This practice necessitated the submarine being on the surface, and so gave a merchant ship defensively armed a chance of replying to the gun-fire and of escaping, and it also gave armed decoy ships a good opportunity of successful action if the submarine could be induced to close to very short range.
The form of attack on commerce known as "unrestricted submarine warfare" was commenced by Germany with the object of forcing Great Britain to make peace by cutting off her supplies of food and raw material.

It has been acknowledged by Germans in high positions that the German Admiralty considered that this form of warfare would achieve its object in a comparatively short time, in fact in a matter of some five or six months.
Experienced British naval officers, aware of the extent of the German submarine building programme, and above all aware of the shadowy nature of our existing means of defence against such a form of warfare, had every reason to hold the view that the danger was great and that the Allies were faced with a situation, fraught with the very gravest possibilities.
The principal doubt was as to the ability of the enemy to train submarine crews with sufficient rapidity to keep pace with his building programme.
However, it was ascertained that the Germans had evidently devoted a very great number of their submarines to training work during the period September, 1915, to March, 1916, possibly in anticipation of the unrestricted warfare, since none of their larger boats was operating in our waters between these months; this fact had a considerable bearing on the problem.
As events turned out it would appear either that the training given was insufficient or that the German submarine officer was lacking in enterprise.
There is no doubt whatever that had the German craft engaged in the unrestricted submarine warfare been manned by British officers and men, adopting German methods, there would have been but few Allied or neutral merchant ships left afloat by the end of 1917.
So long as the majority of the German submarine attacks upon shipping were made by gun-fire, the method of defence was comparatively simple, in that it merely involved the supply to merchant ships of guns of sufficient power to prevent the submarine engaging at ranges at which the fire could not be returned.

Whilst the _method_ of defence was apparent, the problem of _supplying_ suitable guns in sufficient numbers was a very different matter.

It involved arming all our merchant ships with guns of 4-inch calibre and above.

In January, 1917, only some 1,400 British ships had been so armed since the outbreak of war.
It will be seen, therefore, that so long as ships sailed singly, very extensive supplies of guns were required to meet gun attack, and as there was most pressing need for the supply of guns for the Army in France, as well as for the anti-aircraft defence of London, the prospect of arming merchant ships adequately was not promising.
When the enemy commenced unrestricted submarine warfare attack by gun-fire was gradually replaced by attack by torpedo, and the problem at once became infinitely more complicated.
Gun-fire was no longer a protection, since the submarine was rarely seen.