In January 1917 Germany decides to use its submarines to attack any ship going to Britain.
Germany made a decision in January 1917 that changed the war completely. They decided to launch a campaign of unrestricted submarine warfare.
Unrestricted submarine warfare meant attacking any ship approaching Britain regardless of which country it belonged to - even ships from neutral countries could be targets. The Germans believed this would starve Britain into surrendering.
German commanders knew that this was a risky option, as sinking American ships would probably lead the USA to declare war. Germany hoped that by the time the Americans were ready to send their soldiers to Europe Britain would already have been defeated.
Inside a German submarine.