Sheev Palpatine was the sole engineer of his rise to power. Being a former apprentice to Darth Plagueis the Wise, a Sith Lord who was a master of arcane and unnatural knowledge, he could plan events far in the future and use the Force to foresee the results. He began as a Naboo Senator in the Galactic Republic and used several conflicts he had constructed to gain more power.
The first step in his master plan was to create a conflict between a Trade Federation and the government. Palpatine had, as Sidious, created a plan together with the Trade Federation to invaded Naboo and blockaded trade routes to and from the planet. Doing so forced the planetary leader of Naboo, Queen Amidala, to rush to the Senate on Coruscant. With the help of Palpatine, Queen Amidala pleaded to the Senate for an intervention. The Trade Federation stepped in and started to prolong the discussions in order to delay a decision. Disgusted by the Senate inability to act, Queen Amidala took a suggestion from Palpatine and called for a Vote of No Confidence in the Republic's leadership.
The vote went through and the Chancellor of Senate was voted out of office. Palpatine was nominated to succeed him, and with the strong sympathy for the crisis on Naboo, he won the vote and became the new Chancellor.
Palpatine promised to reform the Republic, to bring order and justice, but did little of this. He did in fact start working on creating a Separatist movement. By corrupting the Jedi Master Dooku and turning him to the dark side, he gained a Sith apprentice and a charismatic leader for the movement. Palpatine did not only use Count Dooku to build a secret army of droidsfor the Separatist movement, but also gave him the task of creating a secret clone army for the Republic.
Once the Separatist movement emerged with its army, it was motioned by senators loyal to Palpatine that the Chancellor should be given emergency powers to deal with the threat. This allowed Palpatine to use the newly made army of clones and send the Republic into a full-scale war. A war that was controlled on both sides by Palpatine and used to ensure he held authority for decades.
His term as Chancellor was supposed to end during the Separatists crisis, but the war allowed him to enact executive decree after executive decree. He added amendments to the constitution that gave him more power and allowed him to circumventing government bureaucracy. The public and the Senate willingly gave up their rights and freedoms in hope that Palpatine would give them security.
To distract the biggest threat to his master plan, Palpatine put the Jedi Order in charge of the clone army. Effectively transforming the Republic's peacekeepers into military commanders. Although the Jedi Council grew wary of Palpatine as he issued decrees that redirected power away from the Senate and the constitution. They failed to see the major Sith threat emerging, as their minds were focused on the war with the Separatist.
Palpatine had plans for the Jedi order. By revealing his true identity to the Jedi Anakin Skywalker, he provoked an attack from the Jedi order. The Jedi knights tried to arrest Palpatine and almost succeed. But because Palpatine had befriended Anakin Skywalker and worked on turning him to the dark side, he was saved by him at the last moment. Although Palpatine survived the attack, he became scarred and disfigured forever by the dark energies that had been deflected back at him during the fight. After the fight, Anakin kneeled before Palpatine. At that moment Palpatine knew had succeeded in turning the Jedi to the dark side and gave his new apprentices the title Darth Vader. Palpatine had worked long and hard on turning Anakin. He had even sacrificed the life of his former apprentices, Count Dooku, to push Anakin a step closer to the dark side.
After the Jedi attack, Palpatine transmitted a coded command, Order 66, to all his clone soldiers. The command identified the Jedi Knights as traitors and should immediately be killed. The clone soldiers turned on their Jedi generals and killed them in cold blood. Only a few Jedi survived Order 66.
The next day Palpatine called for a special session in the Senate. Disfigured he appeared before the politicians and spoke about the treacherous Jedi rebellion and how he narrowly escaped their attack. He assured he would continue routing out the treachery, fight corruption and reform the government into a more powerful and secure institution. Palpatine ended his speech by taking the last step in his rise to power and declared himself Emperor. His speech was meet by a thunderous applause from the Senate.
Palpatine ruled his new empire with an iron fist. He instituted an unprecedented military build-up, eventually disbanded the Imperial Senate, and passed control down to the regional governors and the military commanders. His tyrannical rule would ultimately create the Rebel Alliance that valiantly fought against him. As with all Sith, Vader planned to betray his master and overthrow him as Emperor. Vader made a futile attempt at creating a suitable apprentice to help him defeat Lord Sidious, but it ended in failure.
Palpatine met his end when he was betrayed by his apprentice, Anakin Skywalker. His death marks the end of the Galactic Empire and the beginning of a New Republic. He would later return as a clone in the expanded star wars universe, but was defeated by Luke and his friends soon after his "resurrection."