World War One and unrestricted submarine warfare.
It was during the month of February 1915 that the German Empire declared a submarine blockade of Great Britain. Any ship approaching England is considered a legitimate target.
The U-boats sank hundreds of Allied merchant ships. However, submarines normally attack by stealth. This made it difficult to give warning before attacking a merchant ship or to rescue survivors. This resulted in many civilian deaths, especially when passenger ships were sunk. It also violated the Prize Rules of the Hague Convention. Furthermore, the U-boats also sank neutral ships in the blockade area, either intentionally or because identification was difficult from underwater.
A victim of this unrestricted submarine warfare in May of 1915 will be the RMS Lusitania with the loss of 1,198 civilians, including 128 American lives. This single incident will be the first of many "outrages' which will propel the United States into war in Europe in 1917.
Note: During the Second World War the United States and the allies utilized unrestricted submarine warfare against the Japanese Empire.