Final answer:
The prepositional phrase 'for the war' in the given sentence functions as an adverb phrase by specifying the object of the verb 'expected'.
Step-by-step explanation:
The prepositional phrase in the sentence 'Neither party expected for the war the magnitude or the duration which it has already attained' that also functions as an adverb phrase is 'for the war'.
This phrase is specifying the object for the expectation, thereby modifying the verb 'expected' and answering the question 'expected what?'
While serving as a prepositional phrase, it acts adverbially to give more information about the verb in the sentence. When identifying parts of the sentence, such as prepositional phrases and their functions, it's crucial to notice their relationship with other words in the sentence. In this case, 'for the war' is linked to the action of expecting, illustrating the purpose of the expectation.