Soon, all of the Alliance will cry out for war.
—Vladko Mothma
The Coruscant Intergalactic Spaceport Massacre was a terrorist attack carried out by the Resistance at the Coruscant Intergalactic Spaceport. It resulted in the deaths of over 350 civilians and over 30 security guards. The massacre was carried out by six operatives of the Resistance movement, under Vladko Mothma, the leader of the movement. It was also the deadliest terrorist attack in the history of the Galactic Alliance.
The Resistance was a radical anti-Imperial terrorist organisation dedicated to re-igniting the war between the Alliance and the Empire.
The Resistance carried it out with a spy for the sole purpose of making it look like the Empire was behind it, in order to cause a war between the Empire and the Alliance. Imperial Intelligence had assigned Josef Autem to infiltrate the Resistance, using the alias Alexis Barad. Mothma discovered that Autem was a spy, and killed him as he and the other terrorists fled the scene after the massacre. Autem's body was left by Mothma to be found by the Galactic Alliance.
The Alliance discovered that Autem was an Imperial agent, this and the fact that the attackers had used standard Imperial weapons convinced many in the Alliance leadership that the Empire had been behind the attack. Because of public outcry and anti-Imperial machinations by many Alliance politicians and military personel, the Alliance Chief-of-State, Burbakker Teep was forced to step down & his retirement was followed by a coup d'état by the Republican faction, led by Brenn Mothma, Vladko's older brother.
Brenn Mothma and the Republicans enacted a new constitution, declaring the end of the Galactic Alliance and the birth of the Third Galactic Republic, the office of Chief-of-State was abolished and Mothma declared himself President of the Republic, he then declared war on the Empire, beginning the Third Galactic Civil War.