Massive sounds of war and the roar of countless soldiers crash like ocean waves, capturing the sheer terror of an impending foreign invasion. It is a common pattern among the prophets of Israel to place warnings of disaster for the Israelites right next to descriptions of the downfall of the very nation attacking them. The underlying principle is that the whip God uses to strike His people will ultimately be struck itself [רש"י].
The primary approach among commentators is that this prophecy describes the enormous military camp of Sennacherib, King of Assyria. Composed of many different populations, this army rises to flood the land of Judah and Jerusalem. Taking a different perspective, [שד"ל] suggests the prophecy is directed at the combined armies of Aram and Israel, who waged war against Judah during the reign of King Ahaz. The vision opens with a sharp cry of warning and lament over the chaos and destruction the invading force is destined to cause [רד"ק, מצודת דוד], while also capturing the murmuring sound of such a massive gathering [ביאור שטיינזלץ].
The poetic structure repeats ideas to emphasize the sheer scale of the event [רד"ק, מצודת דוד]. Yet, deeper distinctions exist within these descriptions. Some groups within the army are united under a single political government and are simply vast in number. Others are bound together by shared religion and belief, making them broader and inherently more powerful in their quality [מצודת ציון, מלבי"ם]. There is a similar distinction in the type of noise these camps produce. While the primary approach among commentators views the descriptions of noise as basic synonyms for uproar and chaos [מצודת ציון, רד"ק], [מלבי"ם] explains that one represents the standard, everyday noise of a large crowd, while the other represents a massive, unusual, and truly terrifying roar.
The movement of these massive armies is compared to a mighty, raging current of water. This imagery illustrates how the enemy forces flood the entire land until they reach the gates of Jerusalem. Once there, God will rebuke the army and force it to flee, similar to how He rebuked the waters at the Red Sea [רד"ק, אברבנאל]. Adding a deeper layer to this imagery, [אברבנאל] explains that the nations of the world are like a turbulent sea seeking to wash away the dry land, while Israel is compared to the sand. Just as the powerful ocean waves ultimately crash into the sand and break, the strength and schemes of the nations are shattered and brought to nothing before God. The event concludes with the lingering, resounding echo of this immense uproar [רש"י, ביאור שטיינזלץ].