The Onderonian military was a sizeable military force located on Onderon. It was composed of both the army, naval divisions, as well as three concurrent captains who made up the Captain of the Guard.
The year of formation for the Onderonian military is unknown, but it presumably existed for some time prior to Onderon joining the Republic. During the Mandalorian Wars, Onderon was attacked by an invasion force launched from nearby Dxun, becoming one of the first of many worlds conquered by the Mandalorians. Canderous Ordo, reflecting on his experiences in the Mandalorian Wars, would recall that "The Onderonians were certainly brave, but they were disorganized and stupid". Nevertheless, the Onderonian people only grudgingly accepted Mandalorian rule and an armed resistance was led by royal family member General Vaklu. When Republic won the war and the Mandalorians were forced off Onderon, Vaklu emerged from the conflict a decorated war hero.
During the Jedi Civil War of 3958-3956 BBY, Onderon was never directly attacked by the Sith Empire and instead contributed troops, supplies and resources to the Republic war effort. This proved costly for Onderon, as many of the young men who left to fight never came back. By the time of the Dark Wars five years later, Onderon had seen more war in its fifty years of membership in the Republic than it had on its own in the past one thousand years. The military's chief, General Vaklu, was foremost among those Onderonians who wished to secede from the Republic and restore their independence.
The Onderonian military became a constant part of the power struggles between the two royal family cousins, with Vaklu issuing orders revoking civil rights for anyone labeled a "Republic spy". Soldiers supportive of the Queen had to be careful, as they could be fired if they questioned General Vaklu. The navy was forced to do extensive, time-consuming searches of each and every vessel attempting to enter Onderon's spaceport and a brief but intense battle broke out between armed merchant vessels and ships of the navy as Colonel Tobin ordered them to fire on the freighter Ebon Hawk as it arrived.
Onderon separatists during the Second Battle of Onderon.
With the tension constantly building, it was only a matter of time before a breaking point was reached. That happened when one morning in 3951 BBY, General Vaklu appeared in the Council of Lords and pronounced Queen Talia guilty of treason. The support of the Onderonian military was evenly divided. What happened within the space-focused navy is unknown, but the army swiftly turned on itself as soldiers loyal to Talia and soldiers loyal to Vaklu fought for control of Iziz. Remnants of the Sith Empire sent forces to aid General Vaklu, who had secretly allied with them and Vaklu and his right hand man Colonel Tobin led a contingent of men loyal to them to storm the Royal Palace. Just as it appeared he might be able to overthrow Talia and take the throne, Vaklu was interrupted by the Jedi Exile Meetra Surik, who arrived to aid her old teacher and friend, Master Kavar of the Jedi Order.
The Royalist troops barely managed to hold the line against Vaklu and his men and the defeated General surrendered in the Palace throne room. Queen Talia ordered him shot for treason and with Vaklu's death, the secessionist forces of the Onderonian military fell apart. Talia soon after sent a force of soldiers to aid the Republic in the Battle of Telos IV and went on to a long reign, keeping Onderon firmly within the Republic.
In the non-canon alternate path, the Jedi Exile spoke and acted in support of General Vaklu rather than Queen Talia while visiting Iziz. General Vaklu, wishing to break his secret alliance with the Sith, sent Colonel Tobin and a group of loyal soldiers to "kill" the Exile while she met with Master Kavar in the Iziz Cantina. After Kavar fled in the gunfire, Tobin shouted for the fighting to stop and explained that the General appreciated the Exile's help, but had to keep up appearances to make the Sith believe he was still loyal to them. After the Exile accepted the offer of alliance, Tobin had his men resume their attack, forcing the former Jedi to flee Iziz.
Just as the civil war started, Colonel Tobin sent a message to the Jedi Exile and notified her it was time to return to Onderon. Tobin instructed the Exile to plan for an attack on the Sith headquarters at the Tomb of Freedon Nadd on Dxun- to destroy the core forces of Vaklu's soon-to-be-enemies- and for the Exile to come to Iziz to take part in the fighting personally. When the Exile arrived in a Basilisk War Droid, Tobin had difficulty convincing his men there wasn't another Mandalorian invasion. General Vaklu arrived soon after, he met with the Exile and they discussed the plan for assaulting the Sky Ramp and the front of the Royal Palace. The Exile was appointed leader of a small force that would attack the former, while Vaklu would take the main body of his force and hit the latter head-on.
Vaklu's men succeeded in storming the Royal Palace with the Exile's aid, and the Royalist elements of the Onderonian military were defeated soon after Queen Talia's death. Vaklu became King of Onderon and plans began to be made for Onderon's military to work to take control of all of Onderon, not just the walled city of Iziz, "and perhaps more, down the road," in Tobin's words. In this alternate storyline, the Onderonian military sent a force to aid the Republic during the Battle of Telos IV, striking another blow against the Sith and easing the transition from Talia's reign to Vaklu's in the Republic's eyes. Fifty years after becoming part of a multi-planetary coalition force, the Onderonian military resumed fully independent operations as Onderon seceded from the Republic.