Why did the Germans return to unrestricted submarine warfare in 1917?

The flow of war material to France by way of Britain was preventing the German army from gaining an advantage in the fighting. Germany hoped to stop the resupply of the Allies, especially from the United States, and also to deter and demoralize the British, who were resisting them the most successfully in France. The Germans also hoped to relax the British naval blockade of German shipping.