The plan pleased me; I took 12 men from you, one man for every tribe.
Deut. 1:23
Moses knew that despite the fact that G-d had promised His supernatural assistance in conquering the Land of Israel, it was proper to approach the entry into the land in a natural way, for we are not meant to rely on miracles. By preparing ourselves maximally in a natural way, we make it possible for G-d to bless our efforts in a miraculous way, and even elicit Divine favor.
This is why Moses approved of sending out spies to see how the land could be conquered naturally.
—From Daily Wisdom