If you found this article using a search engine, the odds are that what you really mean is “people are”. Although the word people has two meanings (as we will see below), it is almost always used to mean “persons” (the plural of “person”) and is therefore a plural noun. Although the word “persons” exists, it sounds very formal and is used primarily in legal contexts. In spoken English, you will do just fine if you use only “people” with a plural verb:
except in formal contexts, such as a police report)
The same is true for any other verb, not just “to be”:
There is, however, also another meaning of the word “people”, namely “the set of individuals who belong to the same ethnic group”, i.e. something similar to “folk” or “nation”. In this sense, it is usually used in the plural:
The second example is not strictly speaking wrong; it would imply that the individual people saw each other as an enemy. If you mean that different tribes saw each other as an enemy, you must use “peoples”.
In the very same sense, people could be used as a singular noun:
Again, if you mean the whole Maya civilization, not just individuals, you should use “people was”. On the other hand, the second example could be understood as “the Maya people [meaning the individuals] all together formed one of the greatest North American peoples”. Purists could argue that the use of “to be” is not appropriate in such a context, but the sentence would sound just fine to most people. If you want to be on the safe side, use another word like “civilization”, “tribe”, or “population”; these are always used with a singular verb.