Continual concern before forces invaded? (13)

I believe the answer is:

preoccupation

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'continual concern' is the definition.
(an ongoing interest or concern)

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for preoccupation that I've seen before include "A matter that monopolises a person's thoughts" , "Something totally dominating the mind" , "Absorption, engrossment" , "state of France in 1939?" , "Obsession" .)

I've seen this clue in The Times.

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