Das war ein Befehl!
Der Angriff Steiners war ein Befehl!
Those are the lines heard in the movie "Der Untergang." I am not sure why das is used in the first sentence. What I think is if the first sentence is intended to refer to Angriff, then the definite article das should be replaced by der since Angriff is a masculine noun.
Das is used as a definite article, as you mentioned in your question. But, by itself das also has the meaning of that or this.
Das war ein Befehl
Here, das stands by itself. So, it can be understood that it takes the meaning of that or this. With this, the sentence means “That was an order”.
Der Angriff Steiners war ein Befehl!
Here, clearly der is acting as a definite article for the following noun.
It simply means "this/that" as in:
That was an order!
This das is a demonstrative pronoun which stands for the referred noun (group), in this case der Angriff Steiners.
So yes, you can replace the das by this very noun group. But no, you don't have to.
If your question refers to as to why das is used instead of der: the demonstrative pronoun das may generically refer to events, happenings or processes, in this case the utterance of the Steiner's order.
Das refers to a word used before. It is an indefinite pronoun. You can find information about that on the German Wikipedia where this is written:
Darüber hinaus können Indefinitpronomen substantivisch gebraucht werden, wobei sie auch dann zumeist kleingeschrieben werden:
Das gefiel nicht jedem. (substantivischer Gebrauch).
Das ist etwas anderes.
Translated to English:
Indefinite pronouns can also be used substantivial, whereby they will be written lowercase:
Das gefiel nicht jedem. (substantivial use)
Das ist etwas anderes.
You can recognize them by trying to replace das with jenes.
Der is the article for the grammatical gender masculine and belongs to Angriff.
jeder and anderes, not das.
Das war ein Befehl! (That (what just happened) was an order)
If you amend the sentence with a noun you get:
Das (Geschehene, Gesagte) war ein Befehl.
If you replace the nouns with.
Die (Frau, die eben vorbeiging) war eine Asiatin.
Then the das changes to die.
