The most significant development in naval operations in 1917 was Germany's return to unrestricted submarine warfare. By 1917, Germany had developed its U-boats to the point where they had a cruising radius of 10,000 miles (16,000 km) and a surface speed of 25 knots. In April, 1917, U-boats had sunk nearly 900,000 tons. A few more months of losses at that rate would have forced Great Britain out of the war.

In May, however, the Allies instituted the convoy system and the danger began to decrease. From June, 1917, to November 1918, more than 2,000,000 troops from the United States were safely transported to France despite the German submarines.