Why was the torpedo such an effective weapon for the U-boats (submarines)?

The rise of advertising in the 1920s. i am good with seeing how advertisements shape the roaring twenties and how it developed a new way of life through modern days. but i am not sure what the details of advertising in the 1920s were. if possible can you send me links (preferrably credential links) to sources that maybe show like the first company to start advertising through the radio, how advertising benefits (statistics graphs), etc.

Unique among the rulers of this time, persian emperors installed governors in their provinces tolerated local cultures and laws wielded absolute power maintained an extensive army

How else might the black hand have addressed its nationalistic aspirations