Such a state is sometimes referred to as a false peace. The expression may derive from a Bible verse (though it is a transparent metaphor) and is often used in Christian messages:
Jeremiah 6:14
- They have healed also the hurt of the daughter of my people slightly, saying, 'Peace, peace' when there is no peace.
...
To be in a state of false peace is to be in a state of composure — not of indifference, but of composure and satisfaction, in a belief that all is well when all is not well.
An example from a non-Christian source:
... after the Cold War, a realistic background for the rise of democratic peace theory is that people have discovered that after World War II, especially during the Cold War, there are few wars between democratic countries. In fact, this is just a false peace caused by the Cold War. Studies have proved that even among the democracies during World War II, there were many wars ... Yuxuan Song: Is Democracy a Cause of Peace?]
though this is of course far more literal.
The only answer from the suggested duplicate that I feel fits well here is given by ScotM: 'a disaster in the making' (reference: Cambridge Dictionary of American Idioms).
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The persons in such a dangerous position have a false sense of security.