The Children of Israel were meant to enter Eretz Yisrael during the second year after the Exodus. So, Hashem tells Moshe to send out an expeditionary group to survey the land and report back to the people:
And Hashem spoke to Moshe, saying, “Send for yourself men, and let them explore the land of Canaan, which I am giving to the Children of Israel; one man each from the tribe of his fathers shall you send; every one a prince among them. And Moshe sent them from the Wilderness of Paran by the word of Hashem… (Bamidbar 13:1-3).

Tragically, ten of the scouts return with a disheartening report of what they saw, which demoralized the people. As a result, Hashem decrees that the generation of the Exodus should pass away, and that only the next generation would inherit the land.

Rashi’s first comment in our portion (based on Bamidbar Rabbah 16:5 and Tanchuma here) raises a question about the sequence of Torah passages:
“Why is the passage about the spies placed next to the passage of Miriam? —Because she was punished due to the slander that she spoke against her brother. However, these wicked people saw but did not learn the lesson.”

Let’s see what Rashi means. In the section immediately preceding the episode of the scouts, Miriam spoke ill of her brother Moshe to Aharon. Rashi explains there (12:1) that since Moshe needed to be in a mental and spiritual state of readiness to receive prophecy at any time, he had separated from his wife Tzipporah. Tzipporah disclosed this in a moment when her guard was down, and Miriam learned about it. Miriam complained to Aharon that Moshe was overstepping:
“Was it only with Moshe that Hashem has spoken? Has He not spoken with us also?” And Hashem heard. And the man Moshe was very humble, more than any person on the face of the earth was (12:2-3).

Hashem spoke unexpectedly to Moshe, Aharon and Miriam and chastised the latter two for speaking behind their brother’s back. When the cloud of Hashem’s Presence departed, Miriam was left leprous. Moshe prayed for her recovery, but Hashem commanded that she be quarantined outside the camp for seven days. Only then did the people travel, encamping in the Wilderness of Paran.

Miriam’s punishment demonstrates the severity of speaking ill of others (lashon hara). Moshe was very humble, so he would not have been insulted if he had heard Miriam’s accusation. In addition, there is no question that she loved her brother. Consequently, the people waited for her, just as she had lovingly waited by the Nile to see what would become of the baby Moshe in his basket (Shemot 2:4; see Sotah 9b). Also, Miriam’s concern for her sister-in-law’s home life may have been justified. Despite all this, Hashem punished Miriam with leprosy, because she should have confronted Moshe directly.

Rashi says that Miriam’s penalty could have been an object lesson to the scouts to avoid all lashon hara, but they refused to pay heed. And while it is true that the scouts were sent “from the Wilderness of Paran” — the last location mentioned after the episode of Miriam — the Torah did not have to place the two incidents consecutively. Many mitzvot could have been interposed that might have “softened the blow” between these two catastrophic events. Besides, it is possible that Korach’s rebellion (chapters 16-17) may have occurred before the episode of the scouts. Hence, Rashi’s question, and answer.

As the mishnah (Arachin 15a) teaches, despite the fact that the people tried Hashem ten times, it was the sin of the scouts, a sin of lashon hara, that determined their fate to die out in the wilderness and not enter the Land.

The scouts were guilty of speaking lashon hara about the Land of Israel.
It is taught: R. Elazar ben Parta said, “Come and see how considerable is the power of lashon hara. This is seen from the spies: If the spies, who slandered trees and stone were so [punished], then one who slanders his friend all the more so!” (ibid.; see also Tosefta Arachin, chapter 2).

But, as explained by R. Zalman Sorotzkin (1881-1966), in “Oznayim LaTorah,” this a fortiori comparison (kal vachomer) raises a question on Rashi’s comment. It is a novelty to suggest that there can be lashon hara even against the land of Israel. After the episode of Miriam, could not the scouts have been excused for thinking that lashon hara applies only to people, but not to the “trees and stones” of Israel?

R. Sorotzkin’s answer combines two elements. First, Moshe’s humility was extreme to the point of self-nullification:
Greater is that which is said about Moshe and Aharon than that which is said about Avraham. Whereas regarding Avraham it is written (Bereishit 18:27), But I am dust and ashes, regarding Moshe and Aharon it is written (Shemot 16:7), But what are we? (Chullin 89a).

Secondly, the land of Israel, in R. Sorotzkin’s words, behaves “like a living thing.” The land does not tolerate sin, and spews out sinners, of whatever nation (Vayikra 18:27-28). The land is provided with a year of rest, Shemittah (Vayikra 25), and if the people do not observe the Shemittah, then the very land will demand its rest by expelling the people (Vayikra 26:34). Furthermore, one who does not acknowledge his connection to Israel does not deserve to be buried there (see Devarim Rabbah 2:5), because the land is insulted.

Thus, says R. Sorotzkin, the land of Israel, including its “trees and stones,” is a living entity that is less tolerant of an affront than is Moshe. The scouts should have learned that if it is lashon hara to speak against the unassuming Moshe, it is all the more so to speak against the noble Land of Israel.

Each of us is an emissary of the Land of Israel, and we should defend her honor.