Russia is developing an autonomous nuclear torpedo called the Poseidon that Putin says can evade all existing defenses. Experts say if it works, it would give Moscow a new way to target NATO countries and military bases. The torpedo will be equipped on submarines but what happens after it is launched is debated. WSJ explains why Poseidon has been shrouded in mystery and misinformation, how the nuclear torpedo could work and why Russia wants it. Chapters:0:00 Russia’s new nuclear torpedo0:31 Poseidon, explained 1:38 Warhead3:05 How Poseidon deters4:40 Challenges