Allow me to actually give the History of Many-Arrows as it was it late 4e early 5e.
So at this period Many-Arrows is well established and has peaceful if tense relations with all nearby nations and city states.
Its reduction starts when the leader of the Moon Elves trespasses in their territory in pursuit of her husband and daughter after her daughter had killed her brother in a dispute over a magic sword. The elf leader ends up encountering an orc patrol who attack the trespasser. She is unsuccessful in fighting them off, and captured. Fortunately for her the leader of the patrol was Many-Arrows Crown Prince Lorgru, and while fully within his rights to slay her for killing members of the orc patrol and trespassing, decided that releasing her back to her people was the right move. He recognized her and thought this was a smart move for Many-Arrows politically, the Elves would look bad for their leaders trespassing, while Many-Arrows would look merciful and willing to play long games.
Sadly for Lorgru many of the orcs of his country disagreed with his move. While his father Obould XVII supported it being a fellow moderate, the Warlord Hartusk his father's chief rival in the country and voice of the extremist warmongers was using this moment as an example of the royal line's weakness and how they should go to war and kill everyone that snubbed them. Obould was able to keep order but the incident lead to a massive influx of support for the Warlord.
Meanwhile the Drow of Menzobarrenzan were engaging in a big plot, and they decided that part of it required chaos on the surface above the city. And the leadership of the city saw Many-Arrows as the perfect means to that chaos. Drow spies quickly learned of the volatile political situation, and enacted a scheme. First they promised their support to Hartusk taking over the country (they didn't really need to do much else cause he already wanted to do what they wanted). Following this this used trickery and illusion to convince Many-Arrows' High Priest of Gruumsh to support Hartusk. Then they using a sleeping potion on Lorgru to knock him out right before a big feast, and sent in a Drow magically disguised as Lorgru to poison his father and frame him for his death.
With Lorgru framed for the death of the King, and the High Priest supporting Hartusk, the Warlord had very little issue taking control of Many-Arrows and start pushing it towards war with it's neighbors. The Drow further grew his support by getting him the aid of some Frost Giants, alongside the dragon Arauthator "Old White Death" and one of his sons.
Hartusk used the massive armies of Many-Arrows along his Drow, Frost Giant, and Dragon support to sweep across the Silver Marches occupying several major settlements, killing several major leaders and trapping most of the Dwarfs in their holds. However Hartusk was not Obould and had no interest in Kingship or solidifying his gains, and so worked on constantly expansion and started stretching his lines thinner than he should have. This lead to several of his armies being defeated from overextension and the still not defeated Dwarfs would come out of their holds and counter attack the relatively minor forces keeping an eye on them.
Eventually some disastrous defeats at the hands of the United Silver Marches and their allies saw Arauthator abandoning the war effort after the death of his son and feeling he had collected enough loot. Hartusk himself was forced to retreat his lines back to Dark Arrow Keep to regroup and also slaughtered many of his Dark Elf allies for what he saw as their leaders unreliability.
However on the way back to Dark Arrow Keep he encountered a problem Prince Lorgru had returned. He had been released into the wild along with some followers by the Drow before the war started, them thinking he might have some use in the future. Lorgru had won over Dark Arrow Keep's garrison and a lot of other Many-Arrows Orcs and had allowed entry for the Silver Marches forces into the Keep to ambush Hartusk in a pincer attack, resulting in the Warlord's death. Following this the other orcs of Many-Arrows flocked to Lorgru's banner.
After this the Dwarf Kings and other major figures of the Silver Marches met with Lorgru and the Frost Giant leadership to discuss terms, where Lorgru was surprised to learn that Dark Arrows Keep was going to be razed, and Many Arrows kicked out of the Silver Marches in spite of his hope that things could return to more how they were before, however the Dwarves were too angry entertain that idea, and made it clear Many-Arrows were no longer welcome. Lorgru reluctantly agreeing to their terms to Dwarven jeers his last words on the topic being that he hoped one day they would prove themselves worthy of their trust.