を+[する noun] + だ structrure?

Question

So I came across this sentence in an anime,

今の陣形を維持だ

Normally 維持 would take する and make it a verb, but why is it in this case it takes the noun form instead while still clearly being used as a verb with を? I have seen this in a few other instances. Is it grammartical?

Edit: I'm familliar with noun+だ instead of its verb form with する, but what I don't understand is 維持 in this sentence is supposed to enact on another noun with を: 陣形, so shoudn't this be in verb form instead? Could 維持だ be an abbreviation of 維持するのだ?

Answer 1

だ in a short sentence like this often indicates the speaker has just decided/noticed something and is making the listener to take some action right away. This sounds urgent, masculine (男性語) and strong.

• 撤退だ！
  Pull out! / Retreat!
• 終わりだ！
  It's over(, stop what you are doing)!
• こっちだ！
  Come this way!
• 救急車だ！
  Call an ambulance!
• 右腕だ！
  Shoot/attack the right arm!

So 今の陣形を維持だ sounds as if the commander just made up his mind or is reconfirming his decision. The sentence contains a bit of the speaker's emotion as compared to simple 維持せよ.

明鏡国語辞典 explains this as a distinct usage of だ:

だ

❷《終止形で》ある事柄を提示して、行動を促す。「さあ、仕事だ」「飯だ、飯だ」
_{(明鏡国語辞典 第三版)}

Answer 2

Of course it is grammatical. Nouns that can form する verbs are still nouns. 維持 is "maintenance" or "preservation"; 維持する is "to maintain/preserve"; 維持だ is "to be [an act of] maintenance/preservation" - or if you prefer, "<が> maintains/preserves <を>".

So, literally, the sentence means " is to maintain the current formation".

Idiomatically, the "it" here is something like "what needs to get done right now". So this sentence structure forms an indirect command to "hold the line". (This is just another example of Japanese being a high-context language.)