Where were U-boats so dangerous during the war?

1. Which war? WWI or WWII?

2. If the question is regarding WWII, then the British should have easily handled the German sub menace in WWII...AFTER GAINING ALL THAT KNOWLEDGE AND EXPERIENCE FROM FIGHTING THEM IN WWI!

3. Which would make the sub menace not so dangerous after all, except in the very beginning, when people had to catch up to their old ways again.

4. It was publicity and press (news media) that panicked the people.

5. US submarines sank more vessels than U-boats.

6. So the question could also be stated: Why were US subs so dangerous during the war?