There was a time when Demacia and Noxus were sworn enemies.
They had fought many great battles at Green Tooth Peak and the Gate of Sorrows. Although Demacia had won them all, a glance at the map showed that both Green Tooth Peak and the Gate of Sorrows lay on Demacia's borders. Noxus had lost, but those had been offensive campaigns.
Overall, the Noxian Empire was the greater power, and Noxus had long held the upper hand in the struggle for influence across Valoran.
Demacia's elite-soldier policy meant Noxus couldn't easily breach its eastern defenses, but there was no changing the fact that Demacia was always on the back foot. Even in the diplomatic games of Valoran, Noxus—with its fewer scruples—often came out ahead.
However, after its Shurima strategy yielded little and its war in Ionia ended in defeat, Noxus's internal problems grew increasingly apparent. Demacia, for its part, had its own troubles because of the unequal treatment built into its ban on magic. The two giants had been sinking together, but Lux's sudden rise changed everything.
At the Battle of Tobicia, Swin had staked everything, only to be defeated and killed after successive betrayals by the Black Rose and Rammus. The Noxian Empire then splintered into pieces, and from that point on, the fates of the two nations diverged completely.
Before marching out, Darius had already prepared himself mentally. Demacia had suffered a civil war, but at least the country had remained intact. As things stood, Noxus could not compare with Demacia in either economic strength or military might.
So, in this clash between the two nations after so many years, Darius had been very careful to put himself on the weaker side. Rather than fight head-on, he had chosen a pincer maneuver, hoping to win despite his disadvantages.