In his comments at the beginning of the parsha, Rashi asks why the story of the meraglim is told immediately following the incident of Miriam. He also comments that when Hashem instructs Moshe, “Shelach lecha,” or “Send for you,” He meant, “Send of your own volition. I am not commanding you; if you want, send. For Benei Yisrael came and said, ´Let us send men before us… ´ and Moshe consulted with the Shechinah. He [Hashem] said, ´I told them that it [the land] is good; I will thus give them room for error in the matter of the spies, in order that they do not inherit it.´” The Ramban asks on this explanation of Rashi, if, indeed, the request for a scouting mission itself constitutes a sin, then Moshe himself sinned, for, after all, the idea to send spies “found favor in his eyes” (Devarim 1:23). And why would he tell the spies to determine, “Is it [the land] good or bad,” after Hashem had already promised that it was a good land?
To explain, let us first cite the following comments of the Or Hachayim: “It is possible that the intent of those who sent them was evil, and this engendered evil in the messenger, such that the messenger became like the dispatcher – the evil qualities that he did not possess earlier developed within him. Thus, although they were righteous when they first embarked on their spy mission, this negative quality rose within them by force of those who sent them.”
As we know, evil among the people at large causes a lowering of the level of their leaders. We see this later in the parsha, when the Torah discusses the special sin-offering required when the Sanhedrin issues an incorrect ruling regarding a severe prohibition, thus leading the entire nation to sin (15:24). How is it possible that the Sanhedrin, the most righteous among the nation, will err with regard to a severe prohibition (one punishable with “karet“)? The answer is that the sins of the people cause the leaders to come down from their high level, to the point where they can even forget an explicit halacha in the Torah.
The “Tzeror Hamor” describes the process that Benei Yisrael underwent, entrenched in the mire of sin, proceeding from one wrong to the next. From their very first embarkation from Sinai, they regressed; first with the “Mit´onenim,” then complaining about the manna, and so on. It is possible to explain this deterioration on the basis of the principle, “aveira goreret aveira” – that one sin leads to another. First it was only the “assafsuf,” the lowest elements of the nation, that rebelled against Hashem; then the masses at large sinned, and thereafter it was Miriam and Aharon who sinned by speaking lashon hara against Moshe. The Gemara in Masechet Arachin writes that when lashon hara is spoken about an individual, it reflects a spiritual defect in that individual. Thus, although Moshe was a tzadik, he had fallen somewhat from his stature. Thus, having fallen from his level, when Benei Yisrael request a scouting a mission, rather than scolding them Moshe consults with Hashem, as the Ramban notes within Rashi´s view, that this constituted a sin of sorts on Moshe´s part.
This also explains the juxtaposition between the story of Miriam and Aharon and the incident of the spies, which teaches us that through the sins of the nation even the greatest of the tzadikim can fall from his level.