The poem begins in medias res (literally, "in the middle of things"). The Assyrian king Sennacherib has been leading his army on a campaign through the Kingdom of Judah and is about to lay siege to Jerusalem. The poem starts off with Sennacherib visible in the distance. Cutting through the lead-up to this event, the omniscient narrator instead chooses to place the reader at the start of what promises to be a climactic battle.
Starting a poem in medias res heightens its sense of tension and excitement. It sweeps the reader up in the military fervor of the impending battle. In contemporary terms, this is a very cinematic move: it's as if the poem opens with a shot of the charging Assyrian army. In more classical terms, this technique is borrowed from the epic tradition inherited from Greek and Latin poets. For instance, The Iliad begins in the middle of the Trojan war, rather than at the start of it. By beginning this way, the poem establishes certain expectations for what's going to happen. A reader in 1815, when this poem was published, would have been accustomed to poems invoking the epic tradition, and would have thus expected a series of scenes detailing military valor. The poem's meter further emphasizes the military feel of this first line. The anapestic tetrameter (da da DUM | da da DUM) has a galloping feel to it that mimics the charging of horses and lends the poem an irresistible momentum.
At the same time, a contemporary reader, familiar with the Christian tradition, would also have known what's about to happen: God destroys the Assyrian army; there are no scenes of military glory. Furthermore, by comparing Sennacherib to a wolf and the Hebrews to a "fold," or a group of sheep, the poem treats this less as an encounter between two armies and more as a potential slaughter. The comparison to sheep also calls to mind the Christian use of the word "flock" to describe a congregation. This simile, then, does a lot of work. It establishes the good guys (the Hebrews) and the bad guys (the Assyrians). It makes the reader root for the underdog (the Hebrews) and frames those underdogs in Christian terms. This latter move is a radical choice because these are not Christians but Jews, and Jews had experienced (and continued to experience) a long history of persecution and discrimination in England.
So, from the very first line, the poem messes with a Christian reader's expectations. One might expect epic battle scenes to come, except the astute reader knows that God is going to intervene before the battle gets underway. One might also expect a racist portrayal of Jews, given the culture in which this poem was written, but instead the Jews are talked about as if they were Christians.