Unrestricted submarine warfare is the practice of using submarines to attack and sink all forms of enemy shipping, whether they are military or civilian. It is most closely associated with the First World War when Germany's decision to use USW brought the US into the war and led to their defeat.
1.) They had the French on their side.
2.) Guerilla Warfare.
Guerilla Warfare is a sort of petty type of combat - they ambush, use sneak attacks, etc.
Answer: a way to solve disputes between countries before they erupted into open warfare.
Explanation: a way to keep piece before war