The following sentence looks innocent enough:


It is not only imperative to defend oneself but also to strike preemptively.


But its well-formedness is only deceptive. In the eye of the professional, ‘not only’ should find more agreeable neighbours as in:


It is imperative not only to defend oneself but also to strike preemptively.


Eric Partridge, in his influential Usage and Abusage, warns against such misplacing of ‘not only’ (3rd ed., p. 216). Similar grammatical constructions should follow both ‘not only’ and ‘but also’. For example,


The life of an ascetic requires not only self-discipline but also self-denial. [nouns]


The boy has not only butchered the insects but also tortured the animals. [verb phrases]

The expression is equally valid and effective with ‘… not only … but …’:


Public health concerns us all not only because viruses are mutating but because we live in such proximity to each other.


Readers should always take heed of Fowler’s advice on tiptoeing around ‘not only’. He says, 


Not only out of its place is like a tintack loose on the floor; it might have been most serviceable somewhere else, and is capable of giving acute and undeserved pain where it is. (A Dictionary of Modern English Usage, 2nd ed., p. 397)


Editor
www.iso.cuhk.edu.hk/english/features/style-speaks/index.html