Following the failure of the spies, the Israelites suddenly decide to take the initiative and march into battle. Moses issues a severe warning, confronting the people with a grim spiritual and military reality. He urges them not to advance to fight for the land [ביאור שטיינזלץ]. Specifically, he warns them against climbing the mountain, a place where physical danger is highly real and immediate [העמק דבר].
Moses bases his warning on two connected reasons: spiritual rebellion and physical danger. First, he addresses the spiritual crisis, noting that God is no longer among them. Launching a military campaign in this state is not an act of bravery, but rather another act of rebellion against God [אור החיים]. The people might mistakenly believe that showing a willingness to fight and sacrifice themselves will somehow awaken God's care and result in a miracle. Moses quickly shuts down this false hope [העמק דבר]. He makes it clear that without God's active presence, there is absolutely no chance of victory on the battlefield [ביאור שטיינזלץ].
The second reason focuses on the harsh physical consequences of their actions and the reality of facing their enemies [אור החיים]. Moses pleads with the Israelites not to bring destruction upon themselves. He urges them to avoid taking actions that will inevitably guarantee their downfall [ביאור יש״ר]. Marching forward now would only result in a bitter, completely pointless defeat [ביאור שטיינזלץ].