Adam Clarke on Joshua 1:3

The sole of your foot shalt tread upon - That is, the whole land occupied by the seven Canaanitish nations, and as far as the Euphrates on the east; for this was certainly the utmost of the grant now made to them; and all that was included in what is termed the promised land, the boundaries of which have already been defined. See Deu 34:1-4, and see Jos 1:4 (note) below. It has been supposed that the words, Every place that the sole of your foot shall tread upon, were intended to express the ease with which they were to conquer the whole land, an instance of which occurs in the taking of Jericho. It was only their unfaithfulness to God that rendered the conquest in any case difficult.