The peaceful, idyllic world of Alderaan was destroyed in an instant by the planet-shattering laser of the Death Star, the greatest atrocity of a Galactic Civil War that offered no shortage. The demonstartion was meant to cow the Empire's restive worlds and keep them in line, but the Death Star was destroyed days later, and Alderaan became a reallying cry for individuals, systems, and species now determined to resist the New Order.
Alderaan formally adopted pacifism after the Clone Wars, ridding itself of all major weaponry. Cynics noted that this seemingly idealistic move made it easier for the planet and its respected leader, Bail Organa, to actively oppose the New Order in the Senate--all in the name of the finest traditions of the Republic, of course. Alderaan finally crossed the line between opposition and treason when Bail's succusoor, his adopted daughter Leia Organa, was caught receiving secret transmissions from Rebels above the planet Toprawa. Leia was interrogated and her homeworld destroyed byt he Death Star before her eyes.
Alderaan was one of the two inhabited worlds of the system of the same name. Delaya being the other.