He thought the place itself a good position for encamping; and it seemed likely that such an important part of the state as the three thousand heavy infantry of the Acharnians would refuse to submit to the ruin of their property, and would force a battle on the rest of the citizens.

On the other hand, should the Athenians not take the field during this incursion, he could then fearlessly ravage the plain in future invasions, and extend his advance up to the very walls of Athens.

After the Acharnians had lost their own property they would be less willing to risk themselves for that of their neighbours; and so there would be division in the Athenian counsels.
These were the motives of Archidamus for remaining at Acharnae.
In the meanwhile, as long as the army was at Eleusis and the Thriasian plain, hopes were still entertained of its not advancing any nearer.

It was remembered that Pleistoanax, son of Pausanias, king of Lacedaemon, had invaded Attica with a Peloponnesian army fourteen years before, but had retreated without advancing farther than Eleusis and Thria, which indeed proved the cause of his exile from Sparta, as it was thought he had been bribed to retreat.

But when they saw the army at Acharnae, barely seven miles from Athens, they lost all patience.