A difference of race (1) is also conducive to civil war, until they acquire a common spirit: just as a city (2) cannot come into being by a chance multitude, nor in a chance length of time: this is why, whoever has accepted co-settlers either in the beginning or later (3), most of the time they revolted.Notes:
στασιωτικὸν δὲ καὶ τὸ μὴ ὁμόφυλον, ἕως ἂν συμπνεύσῃ: ὥσπερ γὰρ οὐδ᾽ ἐκ τοῦ τυχόντος πλήθους πόλις γίγνεται, οὕτως οὐδ᾽ ἐν τῷ τυχόντι χρόνῳ: διὸ ὅσοι ἤδη συνοίκους ἐδέξαντο ἢ ἐποίκους, οἱ πλεῖστοι διεστασίασαν
1. A. is not speaking of a difference of race (τὸ μὴ ὁμόφυλον) in the modern sense of continental groups; his examples following this passage all involve different Greek peoples who were involved in a common settlement. Indeed, the very word translated as "conducive to civil war" (στασιωτικὸν) refers to inter-Hellenic conflicts (cf. Plat. Rep. 5.470c, where the terminology of στασιάζειν is applied to "unnatural" inter-Hellenic conflicts, while war itself (πόλεμον) is reserved for inter-ethnic conflicts.)
2. Note that by city (πόλις), A. is not speaking only of the physical entity of a city, but rather its political identity, i.e., a city-state. A physical city can certainly exist with a "chance multitude", but A. warns that a city-state cannot, until a sufficient length of time has transpired for the different elements in it to acquire a common spirit.
3. A. uses the words συνοίκους and ἐποίκους. The first one denotes those who live together, and is usually used for those who participate in the act of foundation of a city-state. The second term denotes those who are later added to the population, after the city has already been established.
Aristotle's emphasis on common descent (ὁμόφυλον) as a preventive factor against war seems strange by present-day norms, especially since he is speaking of the co-existence of different sets of Greeks, rather than the co-existence of different ethnic groups, or even different races.
This is why he does not suggest that the possibility of peaceful cohabitation is inherently impossible, but that it takes a long time, until the constituent elements "acquire a common spirit" (ἕως ἂν συμπνεύσῃ).
It appears that Aristotle did believe in inherent differences between nations (cf. Aristot. Pol. 1.1252b, 7.1327b) which would presumably make the possibility of acquiring a "common spirit" even more remote, if not impossible. States with a Greek and non-Greek citizen body were unheard of at his time, so the issue does not seem to have even crossed his mind.