domino effect

noun2

1. •
   The situation in which one event sets off a chain of additional events.

   • While the move has, for now, calmed fears of a repeat of a 2008-style domino effect, the merger has left Switzerland with a new megabank on Paradeplatz square. — 2023
2. •
   (historical) The theory that, if South Vietnam became communist, it would be followed by Cambodia, Laos, and additional countries in Southeast Asia and beyond.