or revenge

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Order 66 was an event at the end of the Clone Wars in which the clones of the Grand Army of the Republic turned against their Jedi commanders and killed them, bringing about the destruction of the Jedi Order. The event was triggered by Protocol 66, an order implanted into the clones by the Sith Lords and Kaminoan scientists who created the clone army. The protocol caused clone troopers to violently lash out at the Jedi and view them as threats, and was controlled by a bio-chip implanted into the clones during their creation; the chip could control the behavior of the clones and ensure total obedience. The conspiracy, led by Darth Sidious secretly Supreme Chancellor Palpatine and Count Dooku, was nearly exposed when the chip belonging to clone trooper Tup malfunctioned, but all who learned of the conspiracy were killed. (x)

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With the Jedi Order overextended, spread thin across the galaxy, each Jedi is alone, surrounded only by whatever clone troops he, she, or it commands. War itself pours darkness into the Force, deepening the cloud that limits Jedi perception. And the clones have no malice, no hatred, not the slightest ill intent that might give warning. They are only following orders. In this case, Order Sixty-Six. Hold-out blasters appear in clone hands. ARC-170s drop back onto the tails of Jedi star fighters. AT-STs swivel their guns. Turrets on hovertanks swung silently. Clones open fire, and Jedi die.
All across the galaxy. All at once. Jedi die.