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Describe how the German policy of unrestricted submarine warfare was different than how they had fought with ships before 1915.

Unrestricted submarine warfare was first introduced in World War I in early 1915, when Germany declared the area around the British Isles a war zone, in which all merchant ships, including those from neutral countries, would be attacked by the German navy. Imperial Germany announces that it will resume unrestricted submarine warfare. This meant that German submarines would attack all and any ships in the Atlantic that they deemed to be supporting the Allied war effort.

Explanation:

It made them very hostile and they would attack all allied war efforts.