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He thought that military action was still a long way down the line.

Is the PP "down the line" a complement in the NP "a long way down the line"?

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I think it is.

+1

I'd say that the NP "a long way" modifies the PP "down the line".

NPs indicating extent commonly modify PPs headed by spatial and temporal prepositions.

We live [two miles / a few minutes' walk beyond the post office].

It happened [ten minutes / a long time after the accident].

We had to leave [a few minutes before the end].

Comments  
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Thank you for the reply.

BillJI'd say that the NP "a long way" modifies the PP "down the line".

If the NP "a long way" modifies the PP "down the line", then, as I understand that, "a long way" is an optional dependant of the PP "down the line".

Is my understanding correct?

Yes, modifiers are always optional items.

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