German submarines, or U-Boats, threatened Canadian merchant ships carrying troops and supplies to Britain, whose war effort depended on this support.
Why U-Boats?
Germany, blockaded by Britain’s superior navy, turned to submarines in an attempt to sever Britain’s naval lifeline across the Atlantic. Germany had a powerful navy, but was cautious about losing it in a major naval engagement. Even though the German navy sank more British warships than it lost at the Battle of Jutland (31 May to 1 June 1916), the Germans retreated back to their ports and were effectively bottled up there for the rest of the war. It fell to the U-Boats to harass the Royal Navy, and the easier targets of merchant ships that were forced to cross the Atlantic.
First Phases of Submarine Warfare
In February 1915, German U-Boats began to attack all merchant vessels in British waters. German U-Boats typically allowed the crews of the ships to disembark before the vessel was sunk, usually by deck gun fire instead of torpedoes, as U-Boats carried a limited number. But the war at sea soon lost its chivalrous nature. On 7 May 1915, the civilian ocean liner Lusitania was sunk, resulting in 1,198 deaths, including many Americans. The neutral United States almost went to war over the incident, and the German high command ordered that U-Boats desist in attacking merchant ships with no warning, which came into effect in September 1915.
But this new cautious policy did not result in enough ships being sunk. U-Boat attacks intensified, although Germany still feared bringing the United States into the war. A year of intermittent attacks, combined with confusing, self-imposed rules by the German Admiralty on U-Boats that required them to surface when confronting large liners in order to determine nationality, proved cumbersome and dangerous to the U-Boats.
Unrestricted Submarine Warfare
With no end to the war in early 1917, Germany returned to unrestricted submarine warfare on 1 February 1917, where its primary aim was to sink all vessels supplying the Allies, regardless of whether the country in question was at war with Germany.
This new unrestricted submarine warfare campaign was partially responsible for bringing the United States into the war on the Allied side in April 1917. The Germans had gambled that unrestricted submarine warfare would win the war by strangling Britain before the full might of the United States would turn the tide. It almost succeeded. With hundreds of ships sunk over the first half of the year, the British Admiralty predicted the possible loss of the war on 20 June unless the U-Boat campaign was stopped.
Convoys for Protection
In response to the U-Boat attacks, Allied merchant ships sailed in groups, called convoys, escorted by warships. The convoys were harder for U-Boats to find and attack, but the U-Boats still posed a terrifying threat. By the end of 1917, 3,170 Allied and neutral ships, totaling nearly six million tons, were sunk. But the new convoy tactics, when combined with limited air support near the coasts, and an increase in Allied war vessels, allowed the all-important logistical lifeline to continue across the Atlantic and sustain the Allied war effort.