This is an excellent question and I apologize for my tardy reply!

To start with, the Germans utilized a campaign of unrestricted submarine warfare at two different points in the war, during 1915 and from 1917 until October 1918. These periods are what are often thought of when someone thinks of submarine warfare during the First World War. During these periods German submarines were sinking any vessel that weas trading with the United Kingdom without having been boarded and their cargo checked. But remember, the Germans were not the only ones with a submarine fleet and that other nations were utilizing them as well, although often in different roles than as unrestricted commerce raiders. The British and Russians, for instance, had a restricted submarine campaign in the Baltic Sea where they boarded vessels trading with Germany and only sunk them if the cargo was considered to be contraband and the crew was off the vessel safely. The Allies also used their submarines to attack the surface fleet of the German navy.

This is a key point. The British were terrified of their surface fleet being sunk or damaged by German submarines, so they often kept it in port to protect their expensive surface vessels which they needed in order to have naval superiority on the surface of the North Sea. The Germans, likewise, kept their High Seas Fleet in port over fears of its vessels being sunk by Allied submarines. The High Seas Fleet was needed to challenge the British on the surface of the North Sea and at the very least exert pressure on it, if it was too whittled down, it would no longer pose a credible threat to the Royal Navy.

The Germans were being affected by the distant blockade that the Royal Navy had instituted. In essence, vessels that were trading with the Central Powers or neutrals with ties to them, would be stopped and their cargo searched for contraband. This, at the very least, did not make the supply situation any better in Germany. In any case, the German people were in dire need of foodstuffs and other goods, and in order to get those for the German people it was thought that winning the war quickly would solve the supply issue.

In order to win the war, they thought it would be most expedient to starve out the United Kingdom, an island nation which relied on trade for many of its goods and foodstuffs. They were instating their own submarine blockade of the United Kingdom. It was hoped that the pressure from dwindling foodstuffs and resource stockpiles would bring the United Kingdom to the negotiating table where they would be willing to concede to the German state. With the United Kingdom out of the war, it was also felt that the French, Belgians, and others on the Western Front would not be able to hold out on their own and would thus also be brought to the negotiating table in favor of Germany.

Thus, the Germans relied on a strategy of unrestricted submarine warfare for two primary reasons: The first was that their surface fleet could not compete with the Royal Navy’s and thus would have difficulty working as commerce raiders. Those ships would be extremely vulnerable to both the British surface fleet and to Allied submarines. This combined with the starvation and supply issues exacerbated by the blockade, which necessitated, from the German perspective, a quick victory as they would not be able to hold out for too long against the Allies who continued to draw on vast pools of resources and men. The Germans had to build up their submarine fleet over the course of the war, they had started the war with about the 5th largest submarine fleet in the world – behind France, Russia, the United States, and the United Kingdom. It was a deliberate policy choice made in light of their strategic situation.

The unrestricted submarine campaign was adopted twice. First, in 1915, but this campaign was dropped after pressure from the United States in response to the sinking of vessels such as the Lusitania and the Arabic. It was adopted again in 1917 with grandiose plans of winning the war in about six months. It was even argued that if the campaign brought the Americans into the war, it would not matter as German submarines would sink every troop transport and thus the Germans would not have to fear a new influx of fresh enemy troops on the Western Front. This is why the Germans utilized submarines so much in the way that they did – they hoped it