Humans colonize a new earth-like planet that they thought uninhabited by sentient life. After a few years, they discover the sentient race living underground in a highly advanced civilization. The natives are peaceful by nature, and their conflicts have never amounted to a war. The civilization is a singular nation with all individuals filling out a role, (no caste system, all individuals are equal and treated the same). After some conflict with the main government from the humans, (more complex than that, but for the sake of a shorter question) the two races become enemies, and a war begins.
Since this race hasn't fought for thousands of years, their only technology for weapons are old relics and captured human weapons (at this point, humans still have projectile weapons, not energy weapons like lasers). The natives have the technology for strong lasers and plasma propulsion, as well as advanced robotics and advanced biotechnology with many of their vehicles and machinery being a melding of organic and mechanical features. With this technology, and well preserved archives of their past conflicts, what would their weapons be like and how fast can they develop them?
Edit: more info- Point 1: I forgot to mention that Earth is gone, or at least uninhabitable, which is the reason the humans left. The starship was built in haste, and therefore had only the essentials to get to the planet and colonize.
Point 2: While having large amounts of resources in the capital, the rest of the human settlements are small villages and towns (the armies are stationed in the capital and some larger neighboring towns). The starship they used is in orbit, but all the power left on it was taken to establish the settlement. after a major collapse in economy and resources due to an alien plant infestation (destroying earth crops and food for livestock), the civilization was reduced to what it is now. The humans had to adapt to husbandry of the alien life instead of earth life, which was not a fast change.
Point 3: As requested, I'll elaborate on the native nature. The natives actually have a sort of link between individuals, not a hive mind, but a sort of overall will of the people. This link isn't in control of an individual, they have their own thoughts and the link does not have a voice or sentience itself, but it has a goal, which is the preservation of the population. the past conflicts (or perhaps war as I'll mention later) were caused by two separate populations having two separate links, with different goals. The peace was formed when the political powers attempted to merge the links, realizing that the best option for preservation was one population, this attempt was successful and the link hasn't split since, which is a large reason they are so peaceful.
Their view towards nature is what brought them underground, seeing that their development and spreading of population would eventually cause them to retreat from the surface, to let nature continue without sentience. Their link doesn't technically extend to the wildlife, but they do feel a sort of connection to them, as many of the wildlife also have primitive versions of the link.
Point 4: As this story is written by two people, me and my friend, we discussed the whole 'conflicts that never amounted to all out war' and decided that could be tweaked to a great war. At the climax of this theoretical war, the previously stated melding of links would have happened. The destruction from the war could have also fueled their moving underground to preserve the surface.