How important was the German foreign policy of unrestricted naval and submarine warfare?
The policy of unrestricted submarine warfare was an important element in Germany's war strategy. It also caused the US to enter the war on the side of the western allies.
It led to Woodrow Wilson's hesitant declaration of war and caused the United States to cut all diplomatic ties to Germany. American propaganda flew like wild fire, enraging citizens and causing them to believe that Germany were blood-thirsty Huns that drew up mass murder without a bat of an eyelash. This also lead into the ideals of anti-German products...or the renaming of them such as Burgers to "freedom steaks" or etc.
The declaration of war on Germany in 1917 was mostly influenced by German naval policy, although American economic interest and Woodrow Wilson's idealism did portray a part.
On February 1915, Germans created a submarine blockade in the British Empire's Ocean's, sinking several belligerent ships, which included the Lusitania. The United States then threatened the German Government causing them to back down. Later in February of 1917, Germany then declared unrestricted submarine warfare threatening the ideals of neutrality, commerce and security. The German U-Boats wanted to sink supply ships in order to starve the Britain islands of their goods. The German's naval policy was the major reason why the United States declared war on Germany. The sinking of the Lusitania was the biggest influence of American in WWI. It was so much of an influence that it was said without the sinking of the Lusitania, the United States most likely would not of entered the war. From Germany sinking the Lusitania, American's grew hatred towards Germany, hence giving Wilson the motivation to declare war on Germany.