Alderaan was a planet near the galactic core in the Star Wars galaxy. It had a representative in the Galactic Senate: Bail Organa at the time of the Clone Wars. Grand Moff Tarkin used the Death Star to destroy Alderaan as a test of the station's superlaser and as the first step in implementing the Tarkin Doctrine.
According to Leia Organa, "Alderaan is peaceful! We have no weapons!" She made this claim when hoping to convince Tarkin not to destroy the planet. Of course, even if true, this claim does not preclude Alderaan having a planetary shield to defend it against attack.