Rather than understanding and obeying God's word with reason ...

After the victory at Jericho (Joshua 6),

those who had spied out the city of Ai said to Joshua,

'Do not let all the people go up; only about two or three thousand men should go up and attack Ai.

Since they are so few, do not make the whole people exert themselves in the labor of the siege' (7:3).

So Joshua sent about three thousand men (v. 4).

They were routed by the men of Ai,

who chased them from the city gate as far as the stone quarries (v. 4).

However, in the second battle of Ai, Joshua selected 30,000 fighting men (elite troops)

and sent them to Ai by night (8:3). While in the first battle, only 3,000 men were sent,

in the second battle, ten times more, specifically elite troops numbering 30,000, were sent.

Why was this the case?  It's not quite clear.

The reason becomes apparent later when Ai was devoted to destruction (v. 25),

as on that day 12,000 men and women, all the people of Ai, were killed (v. 25).

In other words, there were only 12,000 men and women in Ai,

so why did Joshua send 30,000 elite troops?

What's even more perplexing is the fact that God told Joshua to set an ambush behind the city (v. 2).

The reason for this is that when God handed over

the mighty and fortified city of Jericho to the Israelites (6:2),

He commanded them to march around the city once a day for six days

and on the seventh day, to march around it seven times, blow trumpets, and shout (vv. 3-5).

However, when it came to handing over Ai,

which was much smaller than Jericho, to the Israelites

(8:1, 18), God instructed Joshua to set an ambush behind the city as well (v. 2).

God's word, which is not easily understood by our intellect,

ultimately teaches us that rather than understanding it with reason and obeying,

we should obey with a heart of trust (faith) in God and humility

(cf. Prov. 5:6-7).