At one point in the history of the galaxy, all planets belonged to one of four powerful human alliances, each of which controls a quadrant of the galaxy.What these alliances were or how they formed or disappeared is wholly irrelevant, and nor are other species. Each solid planet has been inhabited by humans, with any other life being exterminated. Each was equally matched with the others, and war was seemingly inevitable. By joining with an alliance, a planet would benefit from its trade systems. It would also offer the planet protection from assaults by other planets.
Obviously, with war on the horizon, each alliance is trying to 'recruit' as many planets as they can. Whilst the planets benefit in the short term, their citizens will be conscripted for their alliance if and when war breaks. All the planets know this, but they still choose to side with an alliance. On the surface, they say the benefits outweigh the possible negatives in the future. However with each alliance being so powerful, most planets are surrounded on all sides by an alliance, and feel threatened into joining.
One planet, located roughly around the centre of the galaxy and hence at the intersection between the alliances, refuses to ally with any side. With war looming, they insist their policy is and always be neutrality. Perhaps most crucially, none of the alliances are particularly interested in recuiting the planet. To me, this seems unusual, as it would be in a key strategic location. It remains the only central planet, the only planet even, that is unaligned. In the crucial intersection, the alligience of planets is heavily contested.
It is fairly average, not abundant in resources but not lacking in them. It is a similar size to Earth, but has a population of less than 5 billion. It is, overall, not a particularly hostile environment. For sake of environment, imagine it like Earth, but if Earth had originally been inhabited by other species, all of whch are now extinct. The plant life is similar to the oversized life of the mesazoic era. In the galaxy as a whole, technology is roughly comparable to today's, except with superior spacecraft. Traditional weaponry (guns that fire metal bullets) is still the most favoured, although certain elements of the alliance have shifted their preference to heaver armour and hammers (this is because they can be used to break objects and terrain as well as injure people).
What political or other situation would cause such a planet to remain neutral, and for none of the alliances to want to control it? I begun by considering historical or religious reasons, but I didn't get very far.