Jamieson, Fausset and Brown on Numbers 21:4

4. they journeyed from mount Hor--On being refused the passage requested, they returned through the Arabah, "the way of the Red Sea," to Elath, at the head of the eastern gulf of the Red Sea, and thence passed up through the mountains to the eastern desert, so as to make the circuit of the land of Edom (Nu 33:41, 42).

the soul of the people was much discouraged because of the way--Disappointment on finding themselves so near the confines of the promised land without entering it; vexation at the refusal of a passage through Edom and the absence of any divine interposition in their favor; and above all, the necessity of a retrograde journey by a long and circuitous route through the worst parts of a sandy desert and the dread of being plunged into new and unknown difficulties--all this produced a deep depression of spirits. But it was followed, as usually, by a gross outburst of murmuring at the scarcity of water, and of expressions of disgust at the manna.