第二次ロンドン海軍軍縮会議
On 4 May 1940, in response to Germany's intensive unrestricted submarine warfare, during the Battle of the Atlantic and its invasion of Denmark and Norway, the Royal Navy conducted its own unrestricted submarine campaign. The Admiralty announced that all vessels in the Skagerrak were to be sunk on sight without warning. This was contrary to the terms of the Second London Naval Treaty.