"People fall into two camps" is a common phrase used to describe two groups of people with opposing or different views. Where does it come from?
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The word "camp" comes from the Latin word "campus", which means, essentially a "large open field" – however, the Roman campus had military connotations derived from Rome's Campus Martius, or the "Camp of Mars". It was traditionally a military training ground. In the transition to Old English, the word "camp" lost all notion of a "field", and it was used to mean a "contest, battle, fight, or war", though this definition became obsolete by the mid 1500's, and it gained its meaning as, essentially, a place to camp.
By the late nineteenth century, with traces of conflict still remaining, the word began being used to denote a "body of adherents of a doctrine or cause", and that is where the phrase comes from. It is a military-inspired metaphor, indicating the two opposing sides of an issue are in staunch opposition with very little common ground.
To "fall in", or "fall into", is an idiomatic phrase synonymous with "fit in". In this particular case, however, it may be a reference to another military term, "to fall in", meaning "to line up in a row, standing shoulder to shoulder".
In the phrase "fall into two camps", the word camp has the meaning of
The Online Etymology Dictionary says the use of camp with this meaning dates to 1871:
As to why the people always fall into a camp, I believe it's become an idiom. Maybe another ELU member can answer that.
Camp was used figuratively of people holding a particular view by John Morley, in his The Works of Voltaire in 1871:
We find the idea of two such camps being in opposition to each other by Edward Clood, in his Myths and Dreams in 1885:
Combined with the older "fall into", it becomes understandable as a composed phrase at this point (that is, it isn't an idiom rather than a common turn of phrase, as each part of it works separately according to their separate meanings).
This is largely speculation.
According to The American Heritage Dictionary of Idioms, the phrase have a foot in both camps has its origins in the early 1900s and means:
Going by the corresponding ngram, the expression, fall into two camps, rose in popularity at around the same time and most likely after the expression, have a foot in both camps, did. Furthermore, I believe that the more common version of the OP's idiom is actually, fall into one of two camps. Google suggests that the formulaic fall into one of two [somethings] predates all these phrases by a goodly few decades. I venture to suggest that the OP's expression is a conflation of all these other phrases.