the.com/weak link

means The weakest person or component in a group or system, whose failure determines the failure of the whole.

from Straight from the old proverb 'a chain is only as strong as its weakest link' — a saying that turns iron chain-making into a moral lesson. Chains fail at their frailest link no matter how sturdy the rest are, and by the 1800s people had already stretched that blacksmith's fact into a metaphor for teams, arguments, and systems. The phrase 'weak link' itself crystallized out of that proverb as shorthand for whatever part lets the whole thing down.