But desperation at Germany's failure to win the war on land in 1916 pushed leaders like General Erich Ludendorff to adopt a policy of unrestricted use of submarine warfare. There seemed little choice. German forces were outnumbered on the western front, where 1 50 of their divisions faced 190 Allied divisions. British artillery production was in full swing, and the Germans defending the lines in Belgium and France would soon be facing twice as many British cannon as they had in the Battle of the Somme. Russia's Brusilov offensive in the summer of 1916 indicated only dim prospects for an early victory on the eastern front. "* The prospects of winning the war via the submarine were mixed. On February 1, the Germans began their unrestricted campaign with 111 operational U-boats. Fewer than half were at sea. The submarines of the time required extensive maintenance: thus, for much of the war more than half of the fleet was in home ports for refurbishing at any given time. The army put so many demands on Germany's efficient but badly strained industrial system that the production of new submarines lagged. The shortage of skilled factory labor was a principal problem. Thus, even at the close of the war, due to losses at sea and a sputtering production program, Germany had only 179 vessels; the need for maintaining the vessels kept the number actually on combat stations to about 120.4 For all its deadly striking power, the submarine was a fragile weapon of war. Hit by a single cannon shell, or even rammed by a merchant ship, the thin-skinned submarine was likely to be crippled if not destroyed. Even the concussion from the explosion of a near miss might crack the thin skin of the U-boat's hull. The tactics of striking without warning while the German attacker was submerged came from the U-boats' vulnerability. But this mode of conducting naval operations necessarily put noncombatants trav eling on civilian ships at risk. In this way, the Germans committed the politically disastrous step of provoking neutrals like the United States. The German move to unlimited submarine operations changed the entire nature of the conflict on the ocean in 1 9 1 7, and it raised the strong possibility that Germany would bring the war to a successful conclusion. By the third year of the war, Britain was the mainstay of the Allied side; Britain's defeat meant Germany's victory in World War I; and Britain faced defeat in its most vital effort: at sea.