Monte dei Paschi launched a near-$14 billion offer to buy counterpart Mediobanca, a fresh twist in the consolidation of Italy’s crowded banking market.

Nick Leeson went from being a symbol of everything that is wrong with financial markets to investigating the kind of misconduct he became famous for.

Formally called Banca Monte dei Paschi di Siena, it is widely known as the world’s oldest bank, founded in Siena in 1472. It was long a problem child, hamstrung by poor lending decisions ...