Passage 1.29.9
Ἀθηναίοις δὲ οὐκ ἀνεκτὰ ἐφαίνετο περιυβρίσθαι, καὶ ὡς ἐκομίζοντο ὀπίσω συμμαχίαν ἐποιήσαντο Ἀργείοις Λακεδαιμονίων ἐχθροῖς τὸν ἅπαντα οὖσι χρόνον. ὕστερον δὲ μελλούσης Ἀθηναίων ἐν Τανάγρᾳ γίνεσθαι πρὸς Βοιωτοὺς καὶ Λακεδαιμονίους μάχης, ἀφίκοντο Ἀθηναίοις Ἀργεῖοι βοηθοῦντες· καὶ παραυτίκα μὲν ἔχοντας πλέον τοὺς Ἀργείους νὺξ ἐπελθοῦσα ἀφείλετο τὸ σαφὲς τῆς νίκης, ἐς δὲ τὴν ὑστεραίαν ὑπῆρξε κρατῆσαι Λακεδαιμονίοις Θεσσαλῶν προδόντων Ἀθηναίους.
To the Athenians, being subjected to such humiliation seemed intolerable. Accordingly, upon their return home, they made an alliance with the Argives, who had always been hostile towards the Lacedaemonians. Later, when the Athenians were about to engage in battle at Tanagra against the Boeotians and Lacedaemonians, the Argives came to aid the Athenians. At first, the Argives had the upper hand, but the onset of night obscured the clear outcome of the victory. On the next day, however, the Lacedaemonians prevailed, since the Thessalians betrayed the Athenians.