Aratus saw that the Aetolians were ashamed of openly declaring war on them, as it was so very recently that the Achaeans had helped them in their war against Demetrius,
but that they were so much of one mind with the Lacedaemonians and so jealous of the Achaeans that when Cleomenes broke faith with them and possessed himself of <a class="linkToPlace" target="_blank" href="/place?placename=Tegea&groupId=1011&placeId=1780">Tegea</a>, <a class="linkToPlace" target="_blank" href="/place?placename=Mantinea&groupId=731&placeId=1339">Mantinea</a>, and <a class="linkToPlace" target="_blank" href="/place?placename=Orchomenus&groupId=814&placeId=1470">Orchomenus</a>, cities which were not only allies of the Aetolians, but at the time members of their league, they not only showed no resentment, but actually set their seal to his occupation.
He saw too that they, who on previous occasions, owing to their lust of aggrandizement, found any pretext adequate for making war on those who had done them no wrong, now allowed themselves to be treacherously attacked and to suffer the loss of some of their largest cities simply in order to see Cleomenes become a really formidable antagonist of the Achaeans.
Aratus, therefore, and all the leading men of the Achaean League decided not to take the initiative in going to war with anyone, but to resist Spartan aggression.
This at least was their first resolve; but when shortly afterwards Cleomenes boldly began to fortify against them the so‑called Athenaeum in the territory of <a class="linkToPlace" target="_blank" href="/place?placename=Megalopolis&groupId=745&placeId=1360">Megalopolis</a>, and to show himself their avowed and bitter enemy,
they called the Council of the League together and decided on open war with <a class="linkToPlace" target="_blank" href="/place?placename=Sparta&groupId=660&placeId=1208">Sparta</a>.
This was the date at which the war known as the Cleomenic war began; and such was its origin.
Walbank Commentary