Omens
As the poem's title makes clear, its primary theme is about predicting the future. More specifically, the poem looks at John Brown's death as a sign of the war to come. Brown was a passionate abolitionist who sought to bring about a permanent end to slavery by any means necessary, including violence. The speaker positions Brown's strong anti-slavery stance at the crux of the ideological conflict around the issue. Where Southerners (and Northern moderates) were content to imagine his execution as the endpoint of the clash, Melville makes it plain that it was just the beginning. Brown, as portrayed in the poem, had proved that as long as Northerners allowed Southerners to own other human beings, blood would be spilled. In this way, his actions are framed as predictors of the inevitable showdown between the North and South. For Melville, the central image of the poem is not Brown's hanging, hooded corpse. It is, instead, his wildly unkempt beard. The beard is a reminder of his righteous anger, his radical actions, and the fiery passion for abolition that would continue past his lifetime.
Law
In the poem, Melville explores ideas surrounding the nature of the law and its application. John Brown's execution by hanging is described as following the proper protocol of law. However, as it soon becomes apparent, the role of legality in his death has few simple answers. In a similarly technical manner, slavery was legal in the South during Brown's lifetime and his murder of several slaveowners was obviously illegal. Yet the poem's tragic tone reveals the problems inherent to this simplistic way of thinking. Brown's violent actions were carried out in the name of combating an institution he believed to be in conflict with natural law. For Brown, there was no crime greater than owning another living person and, as such, he saw this moral imperative as more pressing than the written law of the period. In his eyes, slavery was no less than a crime against God. From this viewpoint his execution is not the justified answer to violent crimes, but a hasty attempt to stifle revolution and an omen of the bloody conflict to come.
Radicalism
Related to the questions of law and legality, another theme the poem tackles is radicalism. John Brown's killing of slaveowners and his attempt to start a slave rebellion would be correctly read as acts of radicalism, forceful attempts to destroy what he viewed as the greatest evil of the time. While Melville does not overtly endorse violence, he does portray Brown's rebellion as essentially important and inevitable. Brown's bearded figure is a "portent" because, as the poem shows the reader, his cause would kindle a fire of moral outrage. His radicalism was a last-resort assault on an inhuman legal institution, necessitated, in Brown's mind, by untenable cruelty of its continued existence. What the poem reminds the reader is that what was viewed as radical at the time would now be considered differently. While the violence of Brown's actions might still seem indefensible, his passion for abolition and disdain for slavery would go on to become the norm. The foresight of the poem lies in its imagining of that future. His radicalism was not really an outlying position; it presaged the central conflict of the Civil War.
