Passage 4.29.9
πέφυκε δὲ ἄρα ὡς ἐπίπαν μεταπίπτειν τὰ ἀνθρώπινα, εἰ δὴ Μεσσηνίοις Ἀρκάδας τε ἀντισῶσαι καὶ τὸ ἀδοκητότερον ἔτι ἑλεῖν Σπάρτην ὁ δαίμων ἔδωκεν· Κλεομένει γὰρ περὶ Σελλασίαν ἐμαχέσαντο ἐναντία καὶ τὴν Σπάρτην Ἀράτῳ καὶ Ἀχαιοῖς συγκαθεῖλον.
Indeed, it seems that human affairs are in general subject to sudden reversals, since fate allowed the Messenians not only to find an equal footing alongside the Arcadians, but, even more unexpectedly, to take Sparta. For at Sellasia they fought against Cleomenes, and joining with Aratus and the Achaeans, they brought about the downfall of Sparta itself.