D'varim

The popular question regarding the spies is: why were they wrong in delivering their report about the Land to Moshe and the people? After all, Moshe charged them with checking out the land and its inhabitants and they did just that and reported their findings.

A more specific question can be asked. Parshas D'varim repeats the episode of the spies but with some important enhancements. In recounting the episode, Moshe describes a dialogue that occurred upon the delivery of the spies' report. Moshe explains that after the spies delivered their report, he responded with an attempt to alleviate their concerns by telling them they need not fear the battle since Hashem...hu yilachem lachem - Hashem will wage the battle for us. This dialogue is not mentioned in the original description of the episode in parshas Shelach. If it happened then why is it not mentioned in the original description? If it did not actually happen then why is Moshe stating that it did?

Indeed, the statement by Moshe of "Hashem...hu yilachem lachem" was not mentioned at the time of the report of the spies. It was actually expressed weeks earlier, at the time of the splitting of the sea. At that time, Bnei Yisrael were facing the impassable sea in front of them, the powerful Egyptian army behind them, and the impenetrable desert on either side of them. Fear and despair ruled the day. Moshe, understanding this emotional state, informs the people "Hashem yilachem lachem v'atem tacharishun" - Hashem will wage battle for Bnei Yisrael, while you can simply remain silent and observe. Avi mori explains that the two verbs in this verse can have two different meanings. The verb LaCHeM can mean do battle but it also can mean bread or sustenance. The verb taCHaRiSHun can mean silence (like a cheresh is a deaf mute) and it can mean to plow. At that moment, Moshe Rabbeinu was delivering two essential lessons that are foundations of Judaism, lessons that are not merely applicable to the specific events at Krias Yam suf but lessons that are to be incorporated into the very fibre of Jewish life. The first lesson is the simple translation of the verse: Hashem will always do battle for us and protect us while we can remain silent, in awe and admiration. The second lesson is that Hashem will always provide sustenance for us and always take care of us but we need to plow. We need to put forth effort and toil and then we are assured that Hashem will provide our sustenance, wage our battles, and assure our success.

Moshe did not need to specifically restate these lessons during the episode of the spies. He merely implored them to remember the lessons of Krias Yam Suf.

The sin of the spies was that they misunderstood the charge from Moshe Rabbeinu. Moshe's instructions were not to see if the Land could be conquered, or if its inhabitants could be defeated. Hashem yilachem lachem. Hashem had already assured them that they would conquer the Land. There need not be any fear of any army. Moshe just wanted Bnei Yisrael to feel confident that the Land was one in which they could fulfill their requirements of 'plowing'.

U'mah ha'aretz hatovah hi im ra'ah - is the Land good to be worked and plowed.
U'mah he'arim asher hu yoshev baheinah habemachanim im bemivtzarim - are the cities walled? Can the Land withstand buildings? Can we build homes, yeshivas, synagogues? The spies thought that a land whose cities are fortified is problematic. Moshe thought that a land with fortified cities was a positive as it would imply that after the inevitable victory over the inhabitants of the Land, communities would be built easily.
Hechazak hu harafeh - are the inhabitants strong or weak? When we live there, will we be able to be healthy and grow strong?
U'mah ha'aretz hash'meinah hi im razah - is the Land fertile so that we will be able to support ourselves and our families?

Moshe's charge to the spies was to witness how good and strong the Land and its inhabitants are so that after Hashem yilachem lachem - after Hashem has defeated them for us, then v'atem tacharishun, we will be able to fulfill our end of the deal with personal efforts. The spies thought they were supposed to determine whether or not the inhabitants of the Land could be conquered. This was a lack of faith in the original charge of Hashem yilachem lachem. The battle was already won. They were only charged with assuring the people that v'atem tacharishun, that Bnei Yisrael would be able to perform their duties, thereby, assuring the fulfillment of the first half  of the verse in all its meanings: Hashem waging battles for us and eternally sustaining us.