Strange bird.

There are several important details to keep in mind while evaluating the Siege of Arkanis, and they are as follows:

A siege, variously defined, is “a military blockade of a city or fortified place to compel it to surrender” (Merriam Webster), “the surrounding of a place by an armed force in order to defeat those defending it” (Cambridge Dictionary), or “the act or process of surrounding and attacking a fortified place in such a way as to isolate it from help and supplies, for the purpose of lessening the resistance of the defenders and thereby making capture possible” (Dictionary.com). Wikipedia provides a more detailed definition, adding that blockades are often “coupled with attempts to reduce the fortifications by means of siege engines, artillery bombardment, or mining,” with victories decided either by military force or resource attrition. In the case of intergalactic warfare, in which ground assaults have been rendered more or less obsolete, surface attacks are carried out from the relative safety of starships—a less extreme orbital bombardment, if you will.

The Siege of Arkanis is brief. It begins and ends in the year 5 ABY, concluding with the total surrender of Imperial forces and the annexation of the planet as a New Republic territory. The Imperial Remnants seem convinced of Arkanis’s impending fall by 4 ABY, a full year earlier, suggesting a lack of military might on the part of the planet and/or a lack of faith in the collapsing Empire’s ability to support an Outer Rim world in the midst of Inner Rim conflicts and uprisings in the Core (Aftermath: Life Debt). Given the brevity of the siege, it is highly unlikely that the New Republic achieves victory through resource attrition.

Arkanis, at the time of the Galactic Civil War, is not an economic powerhouse, despite its designation as capital of the Regency Worlds (Wookiepedia). There is “old money” on the planet, and, located along two major trade routes, it likely sees a decent amount of off-world traffic, but its average citizen’s on-world career prospects are limited to service or trade. The planet’s only canon-identified spaceport, Scaparus Port, is a nothing of a seaside town marked by a handful of merchant shops (Wookiepedia). The local industry, fishing, comes with severe occupational hazards due to the presence of carnivorous megafauna in the planet’s ocean(s), and it is not uncommon for fisherfolk to sustain permanently disabling injuries in their line of work. Arkanis Academy, though dangerous in its own right, is an appealing alternative for the planet’s youth that promises purpose, reliable income, and—most importantly—a way off-world, as well as relatively safe employment (e.g. food production, groundskeeping, and maintenance jobs) for locals unable to enroll. Under the Galactic Empire, Arkanis’s most valuable export is military officers, and its most stable economic institution is the Imperial academy.

Given the above, a plausible summary of the Siege of Arkanis might read as follows:

Following the Imperial Remnants’ refusal of terms for the surrender of Arkanis in 5 ABY, the New Republic placed the planet under siege, initiating a blockade of all interplanetary travel and communications alongside a strategy of combined orbital assault and surface-level bombing runs. Strikes targeted infrastructure, grinding on-world commerce to a halt and stranding high-ranking officials of the Regency Worlds—including Empress Leeya herself—in their residences while Imperial holdouts formed a garrison within the fortified Arkanis Academy. Unable to maintain their defense in the face of dwindling munitions, significant damage to the academy grounds, and mounting casualties—and suffering low morale after the disappearance of academy Commandant Brendol Hux—the remaining Imperial forces surrendered after a period of six weeks.

"That's all well and good," you may be thinking, "but this is war we're talking about. What's the problem?"

The problem, as I see it, is twofold:

The Moral Problem: The "Imperial holdouts" referenced in the summary above comprise local bureaucrats, academy instructors, and countless sub-adult cadets. In targeting Arkanis Academy, the New Republic makes both a sound strategic choice and a horrendous moral one; they cripple their enemy's ability to defend the planet, eliminate a link in the Imperial Remnants' personnel supply chain, and knowingly execute children. (I've chosen not to refer to the cadets as "child soldiers" here because, prior to the siege, they have not seen combat.) This is to say nothing of the locals who become collateral damage by virtue of their limited economic opportunities. (Are we to believe that the unnamed mother of Armitage Hux, a "kitchen woman" exploited and abandoned to New Republic bombs by the father of her child, deserves her fate?)

The Political Problem: In Claudia Gray’s Bloodline, readers are introduced to Lady Carise Sindian—Arkanis senator, Elder House scion (There’s that “old money” I was talking about!), and First Order sympathizer. Over the course of the novel, Sindian works to undermine Leia Organa’s influence in the Senate and strengthen the position of the Centrists, a group of politically-aligned worlds longing for “the return of certain aspects of the Galactic Empire” (Wookiepedia); though Sindian is caught and punished for her shady dealings, her constituency votes with the Centrists to secede from the New Republic following the public formation of the First Order in 29 ABY. (You read that right: 25 years after the events of the Galactic Civil War, there is still bad blood between Arkanis and the New Republic.)

Though taking Arkanis by force is expedient, this decision sets the stage for decades of anti-Republican sentiment on the part of both the planet’s richest and poorest citizens. The nobility, affronted by any change to the status quo, resent the New Republic for subjecting them to the indignity of deprivation. (Is this a good time to mention that I headcanon Maratelle Hux née Sindian?) The commonfolk, left reeling in the wake of economic disaster, resent the New Republic for removing the most reliable of their already-limited means to put food on the table. (And, you know, for all the bodies they had to bury. Or whatever it is one does with one’s dead on the space equivalent of a peat bog.)