The First Battle of Onderon was the opening conflict of the greater Mandalorian Wars. For nearly thirty years, after the death of Mandalore the Indomitable on Onderon's moon of Dxun, his successor Mandalore the Ultimate began secretly summoning clans from across the galaxy, building up arms and ships on the moon, working toward the day when a new crusade would be launched. That day finally came, and, in 3,963 BBY, Mandalore the Ultimate sent forth his ships and warriors against the planet beneath them, using the connecting atmospheres to smooth the way for his troops' Basilisk War Droids to attack the walled capital city of Iziz.
When the initial Mandalorian assault was launched from Dxun down onto Onderon, the resulting battle did not last terribly long...in the words of one soldier, "it was like vibroblades cutting through cloth." The people of Onderon had long been noted for their xenophobic refusal to adapt and incorporate cultural and defense characteristics from other peoples, and it cost them dearly when the rain of death began. The defenses of Iziz could not long withstand such a prolonged siege, and the city fell within mere hours.
From Onderon, Mandalore the Ultimate's forces now possessed a staging-point entrée into the Core Worlds and beyond, from whence they could launch a conflict that would spread to engulf the galaxy all the way to the Outer Rim.