Rashi quotes an unbelievable derasha from Chazal that when the meraglim said "chazak hu mimenu" their intention was "kavyachol k'lapei maalah," as if to say the Canaanim were stronger even kavyachol than Hashem and therefore could not be defeated. To think that the dor dei'ah who witnessed the miracles of yetzias Mitzrayim, kri'as Yam Suf, etc should think that the Canaanim could stand up to G-d defies understanding.
Rav Chaim Hirschensohn has a very clever reading of Rashi that gives it the exact opposite meaning of how it sounds k'peshuto. The meraglim understood that everything is in G-d's control. If a nation has power, it's because G- d grants it to them. When they saw the tremendous might of the Canaanim, the meraglim said "chazak hu" -- this strength is -- "mimenu," something that comes from G-d himself. "Ain Baal habayis yachol l'hotzi misham es keilav," G-d will not withdraw his "kelim," the power and might he has given the Canaanim, for our sake.
Shem mShmuel (see also Meshech Chochma, Netziv at the beginning of the parsha) writes that the meraglim's plot needs to be seen in context of the nevuah of Eldad and Meidad at the end of last week's parsha that Moshe would die and Yehoshua would be the one leading the nation into Eretz Yisrael. Moshe was a conduit for the miraculous, but with Yehosha in charge a b'derech ha'teva war would be required. Recall that when Amalek attacked and Yehoshua led the people in battle, there were no overt miracles -- it was a war, plain and simple. On the level of derech ha'teva, would people who had sinned in the misonenim episode, who had sinned at kivros ha'taavah, deserve special treatment from G-d?
Chazal here are not coming to add more criticism on top of the sin of the meraglim, but rather to temper their sin. The meraglim were truly acting lshem shamayim, as they felt that Hashem himself had enabled the nations to remain in control of Eretz Yisrael. The meraglim felt that they were unworthy of geulah at this time.
Rav Hirschensohn ends his piece with a comment no less true today than 100 years ago: "v'zeh yihiyeh mussar l'kol misnagdei hatzionus afilu ha'miskavnim lshem shamayim." Sadly, the lesson still has not penetrated.