How did submarines affect merchant shipping during World War I?

During World War I, German submarines—known as U-boats—profoundly impacted merchant shipping by targeting the vital supply lines of the Allies. Initially, Germany restricted its submarine operations to avoid provoking the United States, but in February 1917, it launched a campaign of unrestricted submarine warfare.123

The effect was catastrophic: in April 1917 alone, U-boats sank 430 ships, totaling 852,000 gross tons. German strategists calculated that if they could maintain a sinking rate of 600,000 tons per month, the Allies would be unable to replace their losses and would be forced to surrender within five months.12

The tide turned when the Allies adopted the convoy system, which organized merchant vessels into protected groups escorted by warships. This strategy, combined with improved antisubmarine technology like depth charges and hydrophones, forced U-boats to risk counterattacks to reach their targets. By late 1917, sinkings dropped significantly, and by 1918, Allied shipbuilding efforts began to outpace losses, effectively containing the submarine threat.142