The Germans honored this convention until 1915, and then switched to unrestricted submarine warfare in which ships would be sunk without warning. And the Germans sank plenty of ships, only to rescind it under American pressure, and then resume it in 1917 as a desperate measure to end a conflict that was bleeding Germany to death.

Was it worth it? The all-out U-boat offensive did sink 880,000 tons of shipping in April 1917 alone and endangered the seaborne trade that Britain depended on. Unfortunately, it also helped U.S. President Woodrow Wilson to persuade Congress to declare war on Germany in April 1917. The intervention of more than a million fresh American soldiers by late 1918 heartened the British and French armies battered by years of war and the devastating German 1918 offensives.

Wilson believed that America should enter the war against Germany, and perhaps he would have achieved this regardless. Foregoing unrestricted submarine warfare would also have sheathed the dagger that did inflict painful cuts on Britain. It also would have postponed the flood of U.S troops that changed the balance of power on the Western Front in 1918.

None of these alternatives would have guaranteed victory, but they at least would have offered Germany a chance. Whether "victory" would have been worth the cost in blood is another question.