Final answer:
U-boats were German submarines used in WWI and WWII that utilized stealth to disrupt enemy and neutral shipping, often in violation of international law. The use against neutral countries aimed to break economic ties with Allies but drew significant international protest.
Step-by-step explanation:
U-boats, or unterseeboots, were German submarines used during World Wars I and II, representing a substantial shift in naval warfare. Their primary advantage was stealth, allowing them to approach and attack enemy and neutral vessels without detection. Despite initially following naval conduct by warning ships before attacks, the advent of armed merchant opposition led to a policy change. Germany began sinking ships without prior warning, a direct contravention of existing international law, thereby defining their actions as unrestricted submarine warfare.
Germany escalated the use of U-boats against neutral countries because they were instrumental in implementing a maritime blockade around Great Britain. This strategy aimed to cut off supplies and impose economic strain, which was crucial since Germany faced a superior British navy. Targeting neutral ships was primarily due to the belief they were carrying contraband to the Allies. The aggressive U-boat campaigns, including wolf pack tactics, were seen as a violation of the principle of freedom of the seas and raised significant objections from neutral nations, particularly after the sinking of ships with American citizens aboard.