To go up against Ai, i.e. to consider and conclude about this expedition of going against Ai; not as if all the people of war did actually go up, which was both unnecessary and burdensome, and might hinder their following design; but it seems to be resolved by Joshua and all the council of war, that the thirty thousand here following should be selected for the enterprise. Either,
1. The thirty thousand now mentioned; or,
2. Part of them, to wit, such as were to lie in wait, as seems most probable, both from the next verse, which limits it to those who were to lie in wait, and from Joshua 8:9, where what is here mentioned only by anticipation is actually put in execution; and it is said of them that were sent forth, that they went to lie in ambush, and did so; and these were only five thousand men, as is expressed, Joshua 8:12. And the only inconvenience of this exposition is, that the pronoun relative them is put without, or before its antecedent, which is left to be gathered out of the following words, which is not unusual in the Hebrew tongue, as plainly appears from Exodus 14:19 Numbers 18:9, Numbers 24:17 Psalms 87:1, Psalms 105:19 114:2 Proverbs 7:8, Proverbs 14:26.