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> もし戦いになったら作戦変更だ

The translation of this sentence in context is "If they start fighting, I'll
change my strategy". My question is, why would you choose to use だ in this
situation instead of する? It's something that seems to be natural and comes
back a lot and I have a hard time grasping. Wouldn't する be more precise? What
is the difference between the two?

**Edit** : After a couple of comments, I will add this because I think it
clarifies well the confusion. I'm trying to put myself in a Japanese setting
where a lot is omitted and the information you have is "change strategy".
Knowing this, and knowing that you're about to potentially take this action,
you can either say "It will be" with だ or "I will do" with する. To a non
Japanese (my frame of reference is English and French), "I will do" seems like
the better option because it **adds the information that it's an action
instead of a statement**. However, it seems like it's the opposite that's
natural in Japanese. How would you explain that?
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question
verbs
Stating or doing in Japanese (です or する)?
Some Japanese nouns can function as -suru verbs. That means that these nouns
can be expressed as in a state of being in reality and as in a state of doing
whatever their assumed function of existing in reality is, which can lead to
the type of grammatical ambiguity that you are experiencing. (To expound on
the second sentence: a waterwheel is often moving. It does not move without
some form of human interaction, but when you paint a mental picture of one, it
may very likely be moving, which means that this state does not need to be
implied with a verb, but can be for increased clarity.)

To me, it is the difference between saying "If they start fighting,
I/they/he/she will change the strategy." (for suru) and "If they start
fighting, there will be a strategical change/change of strategy." (for da)
There is a change in perception or emphasis when comparing these two
variations.

I am not 100% sure of the correctness of this answer, though, as desu appaears
to effectively be a shortening of desuru, with de being used to- seemingly,
from the standpoint of this question, redundantly- a means, time, or location.

It may just come down to intentional omittance for practicality's sake.
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answer
It is gradually known that various grammaticalized copula-ending constructions
play key roles in the Japanese grammar. For this reason, Japanese as a whole
exhibits a strong preference for noun predicate in comparison to average
European languages. (FYI there is [a
book](https://www.degruyter.com/document/doi/10.1515/9783110670844/html)
(didn't read) recently published in English that claims it is an areal feature
in Asia-Pacific, but Japanese still as an extreme example of it.)

More specifically, that type of expression in your example is often classified
as 動作性名詞述語文. 動作性名詞 refers to the bare stem of suru-verbs (勉強, 買い物, and 変更),
which serve somewhat as English gerund. Among their usages, the description is
very clear for your case ([source](http://www.ls-
japan.org/modules/documents/LSJpapers/meeting/161/handouts/b/B-7_161.pdf)):

> a. 未来時における成立が発話時以前に既に確定している事象、または、  
>  b. ある条件の下でその成立が確定するような事象  
>  ((a) an event whose future occurrence is determined prior to the point of
> utterance, or (b) an event determined to occur under a certain condition.)

By using this construction here, you mean it _shall_ (destined to) happen when
the condition is met, or declare the decision practically a done deal to that
effect.

The pragmatic difference between e.g. もし戦いになったら作戦変更 **する** is that, ～する
represents a strong will of speaker towards a future with possible
uncertainty, while ～だ is not that passionate but rather like talking as a
matter of course or already planned.
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answer