Answer: It was a final action leading to the U.S. declaration of war.
Explanation:
The announcement of a total submarine war by Germany followed on 31.1.1917. This was the reason for the US to wage war. With this move, the German authorities broke the international treaty.
Correctly, a year earlier an international agreement was signed in Washington that he would not join this mode of war. The moment the German decides to ignore the deal, the US enters the battle.