What was Germany attempting to accomplish by returning to its policy of unrestricted submarine warfare?

Answer 1

During World War I, Germany's goal in returning to unrestricted submarine warfare in 1917 was, quite simply, to bring the long war to a quick end. Realizing that this decision could bring war with the United States, German leaders calculated that their submarines would end the war before American involvement could make a difference.

Answer 2

Germany resorted to unrestricted submarine use in order to force Britain to surrender first. Prior to this Germany had been deprived of raw materials and food by the British naval blockade.

Answer 3

Because it was an effective tool for harassing Allied supplies, just as it was effective in WWII.