To compromise the security of a top official, how effective is the strategy to compromise individual members (e.g, a bodyguard/servant/cook/etc.) of his entourage by blackmail?
Scenario:
The plotters, in complete secrecy, abduct the loved ones of A (member of the entourage) then communicate with A, threatening to harm A's loved ones if A doesn't do what the plotters want A to do.
Suppose A is compromised and the plot is afoot, are there security measures in place to thwart it?
What are the security measures to avert the scenario or similar ones in the first place?