The Resistance was a small military force founded by Leia Organa to combat the First Order.
The origins of the Resistance, which took inspiration from the Alliance to Restore the Republic, dated to the end of the Galactic Civil War, when the New Republic planetary Disarmament Act in the wake of the signing of the Galactic Concordance with the Galactic Empire.
Organa argued that the Empire, which retreated into the Unknown Regions, could not be trusted to abide by the treaty. When the First Order arose from the ashes of the Empire, Organa urged the Republic to take the threat seriously. Rebuffed and being labelled a paranoid warmonger, Organa was disgraced in the eyes of the New Republic’s Senate following the revelation that her father was Darth Vader.
Ignored particularly by the growing Centrists faction, many of which were in direct collusion with the rising First Order, she formed the Resistance to combat the threat she had discovered.
“We are the spark that’ll light the fire that’ll burn the First Order down.“― Poe Dameron