It is possible that this statement holds true more for the society of the English-speaking world (and probably
Scandinavia) than elsewhere. The principles of English law (with its
Nordic roots, and on which American law is largely based) and the democratic systems of governance in the UK and the USA are based on a fundamentally adversarial paradigm; the role of courts of law is to determine the guilt or innocence of an
accused party (by contrast with
inquisitorial systems of justice which first seek to establish what actually happened and only then determine who if anybody is guilty of something) while the political concept of "The
Opposition" depends on a two-party system; it does not really hold in places where governments are generally coalitions of several parties in a multi-party system, so that the parties not in power are likely to be a disparate collection with little in common.
Nonetheless, in
Italy, which might at first sight seem to be a perfect counter-example, having both a strongly multi-party system (admittedly dating back only to the 1948 constitution) and a tradition of inquisitorial Roman Law, the concept of
dualismo or bipartisan factionalism is as omnipresent now as it was in the
city states of the
Middle Ages: the local struggle between nobles and commoners gave way to the ostensibly geopolitical one between pro-Papacy
Guelphs and pro-Empire
Ghibellines; the eventual supremacy of the Guelphs was followed by a split into black and
white Guelphs and so on via
Montague versus
Capulet until we reach
Coppi versus
Bartali,
Juventus versus
Torino,
democristiani versus
Communists,
Milan versus
Inter; every town and village has another of around the same status nearby against whom there is a permanent rivalry while towns themselves mostly seem split on traditional lines which are unintelligible to an outsider (who can make accidental enemies by going to a dentist or bike shop on the "wrong" side) and only partially congruent with any modern divisions - possibly even the direct descendants of factions which date back to medieval times.
You are either with us or against us