Keep in mind that E-Rantel is an important city because it blocks invaders from marching into the heart of the country. And to make matter worse, if Momon didn't intervene, Khajit would have undoubtedly turned E-Rantel into an undead utopia and tens of thousands of people would have died, just so one man could become undead.The most likely scenario is that the country breaks up into warlord states with every faction attempting to establish a majority power. And without Gazef around, the Noble Faction would have undoubtedly led a civil war against the Royal Faction. Even if Nazarick never got involved with them, Re-Estize Kingdom would have ultimately fallen apart when Gazef would have been killed by the Sunlit Scripture from the Slane Theocracy in the first volume.
With the army in ruins, people almost completely fed up with the nobility, many nobles looking to sell out the Kingdom to Ainz (including Princess Renner), the death of the Kingdom's strongest warrior, and the fact that Ainz already plans to conquer the Kingdom (he could have done it any time but he wants to do it quickly, easily, and with the least amount of bloodshed) the Re-Estize Kingdom is more or less finished. Ainz also had his people take over the underworld of the Kingdom and has been using the Kingdom's crime lords to move food out of the Kingdom to speed up the certain famine to come. he invited Jircniv to Nazarick and the terrified Jircniv immediately sought to make Ainz an ally), challenged the Kingdom to battle over the Katze Plains and the Great Forest of Tob, to which the Kingdom's nobles, against the advice of the warrior captain Gazef Stronoff (the strongest warrior in the Kingdom who knew even he stood no chance against Ainz), accepted while scoffing at the mere upstart magic caster challenging them and sent a force of 200,000 men (mostly peasants and most of the agricultural workforce) to the battlefield where Ainz promptly cast a spell that killed 70,000 of them to use as fuel to summon 5 of the Dark Young that then proceeded to crush another 100,000 or so of the Royal Army on the battlefield. In short order, Ainz gained the backing of Emperor Jircniv (i.e.
That is until The Great Underground Tomb of Nazarick showed up, with its leader Ainz Ooal Gown and his followers. This would have likely succeeded in a few years (The Slane Theocracy had begun thinking about helping the Empire as they saw the empire as the more successful of the two countries). The Baharuth Empire ruled by Jircniv Rune Farlord el Nix had recently set its sights on conquering Re-Estize Kingdom and had annual wars/skirmishes with the kingdom with the sole purpose of depriving kingdom of its agricultural workforce (The Kingdom's army is made mostly from peasant conscripts) during the planting season in order to create a famine, making more unrest, and weakening the Kingdom to being conquered outright. This caused the ruling class to become complacent and the sharp divisions between the upper class and lower class created unrest, resentment, and a thriving criminal underground. As the most secluded of the human kingdoms and protected by many natural barriers, the Kingdom didn't really need to worry about outside invasion.
Even the Slane Theocracy, a nation who hates demi-humans, silently supports Re-Estize's destruction. Asshole Victim: Ainz, the Sorcerer Kingdom, and five other nations all condemn the Re-Estize kingdom for their crimes.Let's put it this way, you could probably count the number of benevolent aristocrats that live in the kingdom on one hand. Aristocrats Are Evil: At least in the Kingdom they are, to the point of stupidity.