“We will be challenged by our enemies, we will be challenged by ourselves, we will be challenged by the hell of war itself,” Mr. Panetta told some 200 Marines, Afghan security officers and troops from other coalition nations at this vast military base in the desert of Helmand Province, abutting Kandahar.

In a sign of the nervousness surrounding Mr. Panetta’s trip, the Marines and other troops who were waiting in a tent for the defense secretary to speak were abruptly asked by their commander to get up, place their weapons — M-16 and M-4 automatic rifles and 9-mm pistols — outside the tent and then return unarmed. The commander, Sgt. Maj. Brandon Hall, told reporters he was acting on orders from superiors.

“All I know is, I was told to get the weapons out,” he said. Asked why, he replied, “Somebody got itchy, that’s all I’ve got to say. Somebody got itchy; we just adjust.”