In the Rebbe's first 
Ma'amar for the Torah Portion of 
Shelach, delivered on the 
Shabat blessing the month of 
Tammuz, 5711, the Rebbe delves into why the Torah states that the decision to send the spies was dependent on Moshe's initiative, and what exactly would have been Moshe's rationale for sending spies given that he knew that the entrance and conquest of the Land of Israel would be miraculous in nature.
The Rebbe explains that the entrance into the Land of Israel first and foremost represented the beginning of the practical 
mitzvot. The Rebbe then explains what it is written in 
Likutei Torah (from the Alter Rebbe) that there are two kinds of 
Tzadikim (righteous individuals): those in the category of 
Leviatan and those in the category of 
Shor HaBor.
Leviatan comes from the word, 
Levayah (accompaniment), and represents connection, and it represents the 
Tzadikim that are involved primarily in spiritual "unifications" (
Yehudim) The Leviathan is a fish, from the sea, which represents the hidden spiritual realm. An example of this kind of 
Tzadik would be Rabbi Shimon Bar Yochai during his 13 years inside a cave with his son Elazar. There, Rabbi Shimon and his son performed the 
mitzvot in a spiritual way only. The 
Arizal was also a 
Leviathan type of 
Tzadik, as was the 
Baal Shem Tov as well.
Shor HaBor represents 
Tzadikim that are involved primarily with the practical, physical 
mitzvot. 
Shor means an ox, and there is a saying that "
Rav Tvuot beKoach haShor," great produce comes with the strength of the ox. There is a special advantage to souls called "beasts of the land," as we see in the Heavenly Chariot (which had images of animals, such as the ox), and how the loftier one's spiritual source, the deeper into physicality it falls. Man is dependent on the food he eats because in truth the food comes from a higher spiritual source than himself.
Even 
Tzadikim in the 
Leviatan category have to perform the Divine service of physical 
mitzvot, unless there is a decree from above, as was the case with Rabbi Shimon Bar Yochai.
In the desert, the Jews' spiritual services was that of 
Leviatan Tzadikim, spiritual in nature. As they were now about to enter the service that involved physicality as well, they needed special assistance from Above, to receive additional strength. That was in fact the main purpose of Moshe sending out the spies, in order to additional strength from Above, in order to be involved in the service of 
Shor HaBor Tzadikim as well.
This specifically had to come from Moshe, who was complete nullified to Hashem, and could sense the higher level of involvement in the physical world. That is why, our sages explain, Moshe longed so much to enter the Land of Israel: to be able to perform the physical 
mitzvot connected to the Land itself.
The spies, although "kosher" individuals, did not have this same level of nullification. They did not sense the holiness of this service, and instead claimed that "the Land devoured its inhabitants" - it made them physical like the Land itself. Two of the spies, however, Caleb and Joshua, understood that it was in fact possible to elevate the physicality of the Land. These two spies were only able to come to this conclusion because they themselves were nullified to Moshe. 
The Rebbe also compares mission of the spies that Moshe sent, compared to those sent by Joshua. Moshe's spies explored the entire Land and the 7 Canaanite nations that lived there. This is compared to the Divine service of the 
Tzadik, to fix the 7 
Middot (also called 
Sefirot) in their essence. The mission of the spies of Joshua, however, are connected to the 
Beinoni (intermediary) only scouted Jericho, which comes from the word for 
Reiach, smell, and represents the rectification of only the outer garments of the soul: its thought, speech, and deed.
[The Tanya teaches that there are two kinds of service: that of the 
Tzadik (completely righteous) and that of the 
Beinoni (intermediary). The heart of the 
Tzadik is a like a Land that has been completely conquered. There is no Other, and therefore there is no struggle. The heart of the 
Beinoni is like a single Land with two competing governments, only one of which is preoccupied with Jewish causes. Neither should the 
Beinoni delude himself and think he is a 
Tzadik, nor should the fact that he is a 
Beinoni make him sad in any way, for it is exactly in this struggle that G-d finds the greatest joy. The 
Beinoni should also not ignore the Other, or even fail to help him in his time of need. On the contrary, he should raise the Other, and bring him along in the service of God. The Land of Israel today is like the heart of a 
Beinoni.]