Where is the noun in sentence Austria issued a deceleration of war against Serbs?

The noun in the sentence "Austria issued a declaration of war against Serbs" is declaration.

Here's why:

* Austria is the subject of the sentence, performing the action of issuing.

* declaration is the object of the verb "issued", and it's the thing that was issued.

* war is a modifier of "declaration".

* Serbs is the object of the preposition "against".