Is there an idiom or word for describing the moment or situation when governments blame some unknown power(s) for bad results caused by their own actions? These governments try to acquit themselves by blaming these nonexistent, external powers.
It is called scapegoating this can be done to a specific person or a vast and unknown group.
Scapegoating (from the verb "to scapegoat") is the practice of singling out any party for unmerited negative treatment or blame as a scapegoat.1 Scapegoating may be conducted by individuals against individuals (e.g. "Hattie Francis did it, not me!"), individuals against groups (e.g., "I failed because our school favors girls"), groups against individuals (e.g., "Jane was the reason our team didn't win"), and groups against groups (e.g., "Immigrants are taking all of the jobs"). A scapegoat may be an adult, sibling, child, employee, peer, ethnic or religious group, or country. A whipping boy, identified patient or "fall guy" are forms of scapegoat.
There is also a mythical creature referred to as the Bogeyman.
a monstrous imaginary figure used in threatening children
Aside from threatening children the Bogeyman makes a great target for politicians warning about threats(real or imagined) from unknown outside forces.
Not immediately in the area of politics, but in insurance-land, if anything cannot be attributed to a legal or natural person or entity, it is called an act of God.
This has no actual religious implications, beyond the sometimes sarcastic side-effect when a company denies responsibility or refuses to pay damages when they consider something an "act of God".
Usually what is meant are things like earthquakes, hurricanes and the like, although I have seen one policy that included under this heading a "collision with a seafaring vessel" (the insured property being located 200 km inland, that would be an act of God) and "a nuclear explosion"...
The word is not commonly used in modern politics, but attributing things to "Providence" was very common in 18th and 19th century America. It means: divine guidance or care. The Puritans believe in Calvinist predestination and brought the idea with them. They founded Mass. Bay Colony to be a "City Upon A Hill" and believed that the US had a "Manifest Destiny" to expand across the continent.
Providence mostly has positive connotations, but if something goes wrong, it was an easy-out to say Providence didn't see fit to grant us this or that.