The Austro-German drive (October-December, 1917) swiftly undid the work of two years of most arduous endeavor.  The Italians were forced back from Gorizia and compelled to surrender mountain positions which had been captured by them at enormous cost.  Back across the boundary they retreated, losing heavily in men and material.  The enemy advanced into the low country near Venice, and it seemed for a time that the city would fall into their hands.  But British and French assistance was sent to Italy, the Italian army recovered its spirit, and a permanent check was put to the enemy’s advance before Venice was reached.  Upon a much shorter but more defensible line the Italians held the enemy at bay in the mountains and along the river Piave (pyah’v[=a]).

[Illustration:  WAR ZONES]

UNRESTRICTED SUBMARINE WARFARE.—­On January 31, 1917, the German ambassador to the United States, Count von Bernstorff, announced to President Wilson that Germany would begin unrestricted submarine warfare the following day, in the waters around Great Britain and France,[4] thus withdrawing the pledge given as a result of the sinking of the “Sussex.”  Three days later the President handed Count von Bernstorff his passports and recalled Ambassador Gerard’ from Berlin, thus severing diplomatic relations with Germany.

During the next six months shipping was sunk at an average rate of 600,000 tons per month, three times as fast as before, and two or three times faster than it was being replaced.  The highwater mark was reached in April, when 800,000 tons of shipping were destroyed.  Unless this loss could be greatly reduced the Allies for want of food and materials would soon have to give up fighting.

But methods were quickly devised to combat the new danger.  The patrols were increased, ships voyaged under convoy of fast destroyers constantly hovering about on the watch for submarines, and other protective measures were taken, so that the submarine menace was soon much reduced.  By September, 1918, the sinkings were only about 150,000 tons a month, while the production of ships, especially in the United States, has increased to several times this amount.

Apparently Germany had waited until she had built a large number of submarines, thinking that by the use of a great fleet of them in a ruthless warfare on shipping she could force a peace within a few months.  In this expectation she was disappointed.  The principal result of the withdrawal of her pledge to this country was the entrance of the United States into the war on the side of the Allies.  Captain Persius, an expert German naval critic, admitted in November, 1917, that the German admiralty was grossly mistaken in its calculations and that Germany had no reason for believing in the decisive influence of the submarine war.