Attack on the Sorcerer Kingdom's Transport

Attack on the Sorcerer Kingdom's Transport was an attack orchestrated by the Re-Estize Kingdom's New Faction on carriages transporting humanitarian aid from the Sorcerer Kingdom to the Roble Holy Kingdom. It would end up being the spark that led to war between the two nations.

Background
Following the Massacre at Katze Plains, the Royal Army had been devastated. Thus, the Re-Estize Kingdom had no choice but to avoid any sort of conflict while they recovered their strength.

Meanwhile, The Sorcerer Kingdom had already begun plotting to conquer the Kingdom; a plan Sorcerer King Ainz codenamed "Candy & Whip." It involved manipulating a noble into organizing a revolt, which would give the Sorcerer Kingdom a casus belli to invade. Before this though, they would earn the citizens' trust to make them more accepting of their "intervention."

While on a diplomatic visit to the Kingdom, Prime Minister Albedo met a low-ranking noble named Philip, who she deemed to be a perfect pawn. To prop him up, she had Eight Fingers gather upstart nobles to form a New Faction, with him as the leader. She also had the criminal organization steal massive amounts of food from the Kingdom to further dwindle supplies while the Sorcerer Kingdom began exporting its own, much cheaper goods.

Contrary to Albedo's plans, however, the influx of food interfered with Philip's plans to inflate his prices, causing him to turn on his benefactors. Together with fellow New Faction members Wayne and Igor, he plotted to rob the carriages transporting the food and sell the contents as their own. While two initially did not want to participate, foreseeing the consequences, they calculated that the Sorcerer Kingdom should only have the manpower to annex regions close to the border in case of a war of retribution, far away from their domains.

In preparation for the attack, Philip drafted 50 peasants of his domain while Wayne and Igor both drafted 75. Philip and his men would attack first, while Wayne and Igor's forces stayed back. Unbeknownst to Philip, this was so the two could minimize their losses.

Incident
The "battle" was over before it even began, as the leader of the Eight Fingers members transporting the carriages, Christopher Olson, believed this to be yet another act of infighting between the organization's divisions; thus, not wanting to risk his and his subordinates' lives over something so seemingly trifling, he gave into Philip's demand to hand over the transport and fled the scene with his men.

Aftermath
Once Philip had secured the transport after its guards had fled the scene, Wayne and Igor arrived and expressed shock at how easy it went. As agreed, Philip would take 80% of the spoils while Wayne and Igor each got 10%. However, the two wanted nothing to do with an incident that could spark a war and offered to have Philp have all of it, though the latter insisted they have their share. Due to all passengers surviving and being Eight Fingers members, the Sorcerer Kingdom quickly learned of the attack. Albedo blamed the organization for losing control of Philip and brought his overseer, Hilma Cygnaeus, in for questioning before Sorcerer King Ainz himself. However, the monarch ultimately led her and the organization off once it became clear they had no involvement in the attack.

Albedo and Demiurge believed the attack to be part of a larger plot. However, when Ainz brought up the possibility that Philip simply had not thought the consequences through, the Floor Guardians immediately sided with their master's "deduction", which ironically ended up being spot on. Regardless, Philip's actions showed him to be an unreliable pawn; thus, with the likelihood of increased foreign intervention, the Sorcerer Kingdom was forced to alter their plans from destroying the Re-Estize Kingdom from within to declaring open war on the nation using the attack as a casus belli.

After receiving the declaration of war, the Kingdom's leadership gathered for an emergency meeting. Given the difference in military strength, they had to sue for peace. Thus, when Albedo visited the Royal Capital, King Ramposa III offered up his own life to defuse tensions. Unfortunately, Albedo declared that they would not settle for vassalization or peace offerings and that they would invade in one month. In reality, though, this was an elaborate ruse and the Sorcerer Kingdom began their invasion immediately.

Despite causing an incident that sparked a war, rather than facing punishment, the New Faction was placed under protection, as it was believed they were [Charmed]. However, they soon experienced their own kind of punishment: as the war with the Sorcerer Kingdom got worse and worse, they found themselves wrought by guilt for causing such a disaster and they finally paid retribution when their former benefactors silenced them to erase evidence of "Operation Candy & Whip."

In the end, Wayne and Igor's calculation ironically ended up being correct in a way: the Sorcerer Kingdom lacked the manpower to conquer the Kingdom, so they destroyed the nation instead.

Trivia

 * The anime only features Philip, Igor, and Wayne planning the attack and its aftermath, skipping over the incident itself.