The systematic military campaign waged by the Israelite armies against Moab was one of total devastation. The invasion left the nation's infrastructure and agriculture in ruins, ultimately leading to a massive siege on the Moabite capital.
The destruction of the countryside was thorough. Soldiers ruined the fertile agricultural plots by filling them with rocks [ביאור שטיינזלץ], with each fighter throwing a specific stone assigned to him [מצודת דוד]. Some suggest that these stones were not simply picked up from the ground, but were actually taken from the walls of the other Moabite cities that the soldiers had already demolished [רש״י].
After devastating the surrounding environment, the battle shifted to the political capital and metropolis of Moab, a large, famous, and heavily fortified city where the Moabite king had barricaded himself. The sheer scale of the destruction meant that all other cities and regions were ruined, leaving only this capital standing. Its walls were constructed from exceptionally massive and strong stones, which prevented them from being easily destroyed like the rest of the country [רד״ק, מלבי״ם, אברבנאל, רלב״ג, מצודת דוד]. Presenting a completely different perspective, [רש״י] explains that the Israelite soldiers actually dismantled the city's walls stone by stone, leaving behind nothing but the clay mortar that had originally bound the stones together.
As the campaign reached its final stage, the heavily fortified walls of the capital required a different military strategy. While standard infantry troops had successfully conquered the surrounding cities, this fortress demanded specialized intervention. The primary approach among commentators is that the attackers were not merely foot soldiers armed with simple hand slings [מצודת ציון, ביאור שטיינזלץ]. Instead, the military deployed heavy siege engines designed to hurl massive boulders, intending to batter and breach the formidable walls [רד״ק, מלבי״ם, אברבנאל].
The relentless, crushing impact of these massive siege stones against the fortress walls drove the king of Moab into absolute despair. After launching a failed attempt to break through the Israelite siege lines with hundreds of swordsmen, he turned to drastic measures, performing an extreme act of human sacrifice right upon the city wall [רלב״ג].