This question is slightly troubling in that, in the translation of Antigone that I am using, I can find no simile describing the entire Argive army.
There is, however, a simile that is used to describe the leader of the Argive army, Antigone's brother, Polynices.
"He set forth against our land
because of the contentious claims of Polynices,(110)
like a sharply crying
eagle flying into our land"
This description (at lines 109 - 112) is offered by the Chorus and implies that the fierce, predatory characteristics possessed by the eagle were also demonstrated by Polynices in the battle to overcome Thebes.
I hope this helps.