As part of their trio of major offensives around the turn of the year 3963 BBY (according to the year system), during the Mandalorian Wars, the Mandalorian Neo-Crusaders launched an invasion of the planet Onderon and Dxun, its moon which was typically held by Mandalorians.
Following Mandalore the Indomitable's demise on Dxun, Onderon's moon, his successor, Mandalore the Ultimate, spent nearly three decades secretly rebuilding the Mandalorian clans from across the galaxy. He stockpiled weapons and ships on the moon, anticipating the day a new crusade would begin. That day arrived in 3963 BBY, when Mandalore the Ultimate unleashed his ships and warriors upon the planet below. Utilizing the shared atmospheres, he facilitated the attack of his troops' Basilisk war droids on the walled capital city of Iziz.
The Mandalorian assault, initiated from Dxun onto Onderon, resulted in a battle of surprisingly short duration. As one soldier put it, "their 'resistance' was laughable. We cut through their defenses like vibroblades through cloth." Onderon's people were known for their xenophobic resistance to adapting and incorporating cultural and defense strategies from other cultures, a flaw that proved fatal when the onslaught commenced. The defenses of Iziz proved unable to withstand the prolonged siege, and the city succumbed within a matter of hours.
With Onderon under their control, Mandalore the Ultimate's forces now had a staging area, an entry point into the Core Worlds and beyond. From this location, they could initiate a conflict destined to engulf the galaxy, stretching all the way to the Outer Rim Territories.