In Antigone, what is the significance of the simile Sophocles employs to describe the army from Argos?

I assume you are refering to the first speech from the
Chorus in this play, which is a kind of victory chant for the success of Thebes against
the invading army of Argos. The army of Argos is described as being like an eagle in
this speech. Note how the Chorus develops this
comparison:

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And he had driven against our
borders,

launched by the warring claims of
Polynices--

like an eagle screaming, winging
havoc

over the land, wings of
armour,

shielded white as
snow,

a huge army
massing,

crested helmets bristling for
assault.

The effect of this
simile is to describe the serious threat and danger that the army of Argos represented.
The comparison of the armed forced to an "eagle screaming, winging havoc / over the
land" serves to emphasise the fear that the army caused in the hearts of Theban
citizens, and the reference to the "huge army massing" again serves to exaggerate the
threat and numbers of the army. This of course makes their defeat at the hands of the
Theban defence all the more glorious. The simile thus serves to describe the very real
threat and danger that faced Thebes, which makes their victory all the more
meaningful.