War brings down whole civilations. Whether it's two small warring tribes seeking revenge on past wrongs, interplanetary arguments over trade routes or full scale intergalactic conflict, it instigates change. It topples governments and raises new ones. Here are some of the most notable examples, from an apparently small planetary issue to a conflagration that consumed a galaxy.
The Biggest Wars
The Great Hyperspace War
This is the first return of the Sith to the Republic. Two Hyperspace explorers, Gav and Jori Daragon, stumbled upon them and hoped for trade agreeements. Ignorant of the sith's relation to dark jedi exiled from the Republic they led them back to Republic Space. Only the Jedi and Empress Teta of the Koros system foresaw the danger, and fought off the Sith, basically destroying the Sith Empire, which had attacked unprovoked.

The Sith War
This began with Onderan a world seeking to join the Republic. But Onderon was home to an ancient Sith, and his legacy still permeated the planet. The Jedi joined the native freedom fighters and Princess in defeating the Sith Supporters. But the conflict was only beginning.
Aleema and Satal Keto, children of rulers from the Empress Teta system sought out the Dark Side knowledge there and overthrew their own government. Meanwhile, Jedi Exar Kun, sought out the Dark knowledge as well. He too, fell to it's power.
War broke out between the Teta system and the Republic. Meanwhile Uliq Qel Droma, infiltrating them, fell to the Dark. He became Exar Kun's apprentice, with the Aleema as his ally and Satal killed. Kun tricked many jedi apprentices into the Sith, setting them to murder their former masters.
The war invaded the Senate itself on Coruscant, destroyed the planet Ossus and finally ended on Yavin IV when Uliq Qel Droma broke free of the dark side and led the Jedi Fleet to Kun's hideout. Kun's body and armies were destroyed, leaving Exar Kun's spirit trapped within the temple ruins.

The Mandalorian Wars and the Sith War
This was a full scale conflict between the Mandalorians and the Republic. The Republic won, but in the process, the two Jedi leaders of the conflict fell to the Dark Side and became Sith. Malek and Revan caused chaos throughout the galaxy when they not only went over, but convinced much of the Republic fleet to join them.

Sith VS Jedi: Brotherhood of Darkness
The Brotherhood of Darkness or of the Sith was a full scale war between Jedi and Sith. It devastated worlds and ended in disaster for both sides.
It lead to reformation of the Jedi Order as well as the new sith "Rule of Two".

The Clone Wars
The Clone Wars were so named for the 'clone army' used by the Republic. They were all clones of one bounty hunter, Jango Fett and created by the Kaminoans on order - allegedly - of a Jedi Master Sifo-Dyes for the Republic. However, the Jedi had no knowledge of them, Sifo-Dyes had been killed about the same time the order was placed and the Republic was still deadlocked on a vote to even create an army to defend against the Separatists.
The decision was taken out of their hands when Obi-Wan Kenobi stumbled over a staging area for the Separatists while pursuing a bounty hunter. He and his apprentice Anakin Skywalker, along with Senator Amidala, were about to be executed. The Jedi staged a rescue. The Separatists responded with overwhelming force using an army of battle droids. Many Jedi died in the Geonosis sands and Jedi Master Yoda arrived just in time, with the clones, to rescue the survivors. With the escape of Separatist leader, Dooku, the war was officially on.

The Secret of the Clone Wars was this: it was a sham. Dooku was Darth Tyrannus, apprentice Sith Lord to Darth Sidious, also known as Supreme Chancellor Palpatine of the Republic. The true goal: discredit and kill as many jedi as possible. The war chewed up resources and gave him more excuse to take emergency powers 'for the good of the Republic'.
As the war came to an end, he ordered the Clones to kill the Jedi, converting Anakin Skywalker to Darth Vader to replace his dead apprentice. The jedi were killed en masse and he declared them traitors and announced they had rebelled against the Republic. And so he reorganized the government into an Empire, with few ever being aware of the true conflict.
The Rebellion against the Empire
The Alliance to Restore the Republic began with three senators: Mon Mothma of Chandrila, Bail Organa of Alderaan, and Garm Bel Iblis of Corellia. They convinced the separate, individual groups rising in dissension to join together for the mutual goal of bringing down the corrupt Empire. It started with very little, but more and more peoples, planets and companies joined as the cruelty and corruption increased. Often aid was given in secret.
The true victory came when the aid of the last Jedi, Luke Skywalker, led Anakin Skywalker to reject his life as Darth Vader and kill the Emperor. Simultaneously, the Rebels on the Endor moon destroyed the shield for the Emperor's under construction battle station. The alliance fleet proceeded to demolish the battle station and the remaining Imperial Fleet scattered.

The Yuuzhan Vong Invasion
The New Republic had finally settled into a war free state. This is when the extra galactic invaders hit. They infiltrated in secret, seeking to divide loyalties and then invaded with a massive fleet. Their goal: conquest of the galaxy. They were there with their families as well as their warriors and meant to stay. The Yuuzhan Vong worshiped gods of pain and enslavement, and they left massive destruction in their wake. They took already fertile, populated worlds and terraformed them, to supply their organic weapons and ships.
The Jedi were the first to recognize the threat. They could not sense these beings in the Force, but the evidence was conclusive. Nonetheless, the Republic was slow to acknowledge the jedi's warnings and other species, like the Hutt's, thought they could bargain their way out.
In the end, it took the invasion of Coruscant, the near total destruction of the Senate and allies like the Hapans and Imperial Remnant, joining the Jedi, to defeat the invaders. The Jedi uncovered secrets of the Yuuzhan Vong past which explained the reasons for their separation in the Force and convinced the lower classes of Yuuzhan Vong to join with them. At this point the New Republic reformed under the name 'Galactic Alliance' and the Jedi Order helped the Yuuzhan Vong find unclaimed space to settle and ways to use their organic technology for the good of the galaxy.

The Corellian Uprising
Corellia, a planet known for it's heroes and smugglers, was agitating for more power. A slew of terrorist attacks on Coruscant were blamed on them, leading to anti-corellian sentiment. War broke out, dragging other systems into secession. This lead to the formation of the Galactic Alliance Guard. Heroes like Han Solo and Wedge Antilles found themselves caught between their homeworld and the government they defended. Most notably, against the wishes of the Jedi Council, Jacen Solo son of Han Solo and Leia, student of Luke Skywalker, was lured to the Dark Side. He became the first new Sith in a generation: Darth Caedus. The Jedi sought to sit out this war, realizing it was due to a communications breakdown. But they could not ignore the growing danger Jacen Solo posed, as he increasingly used torture and terror against Corellia. This brought them into conflict with the Alliance, which still saw Jacen as a hero. This ended tragically for the heroes: Jacen was killed by his twin sister, Jaina. The war ended, but the Jedi, once again, had lost the trust of the government and populace.
Big Wars begin with Small Ones.
Melida/Daan This individual planet never caused external problems. But it's own internal war waged for centuries, leaving an entire middle generation dead. The Melida killed the Daan, the Daan the Melida for slights from past generations. Enter The Young. The children of the world rose up and fought a handful of battles and won their world, with the help of a very young Padawan jedi named Obi-Wan Kenobi and his master, Qui-Gon Jinn.
Phindar This planet was conquered from within by an oppressive regime. Using mindwipes and shortages of basic necessities, they maintained control. This plan was only beginning. Making an Alliance with the Prince of neighboring Gala, they threatened to expand their conquest into the nearby systems. Again, they were stopped by Kenobi and Qui-Gon Jinn.
Nar Shaddaa and the Pirates The world Nar Shaddaa was loosely affiliated with the Hutts, outside the Republic. But slavery was legal there and it's operations were endangering travelers. The Colicoids were members of the Republic, but secretly involved with the slave traders and mine owners. The Jedi again intervened, taking down the Pirate leader and his band and convincing the Colicoids that if they took over the whole operation and operated within Republic law, they would profit. The Jedi involved: Siri Tachi (who had been working undercover), Obi-Wan Kenobi and Anakin Skywalker
Ansion threatened to secede from the Republic when their own internal conflicts threatened to force them into violating Republic law to deal with them. Had it done so, a complicated series of treaties and agreements would have drawn many other systems out with them. The conflict again was settle by jedi. These were Obi-Wan Kenobi and Anakin Skywalker and Luminara Unduli and Bariss Offee The timing was right before the Clone Wars.