The Empire is the most despised regime in Star Wars, and for good reason. It forcefully imposed its dictatorship over the galaxy and held a strong prejudice against any non-human species. While some planets prospered with the Empire's reign, the vast majority were damaged and left under close surveillance. And out of all the planets in Star Wars, Kashyyyk, which housed the Wookiees, fared worst of all, with the Empire treating its inhabitants as nothing but animals.
Compared to the majority of intelligent species, the Wookiees were seen as a little more primitive than most. Their home planet of Kashyyyk houses dense forests and gigantic trees, and much of the planet's architecture is made from wood. And, of course, the Wookiees themselves are animalistic, baring a coat of fur with little clothing and talking in growls and roars.
However, Wookiees are far more intelligent than many give them credit for. As seen during Star Wars: Revenge of the Sith, they combined their woodwork with advanced technology to create flying vehicles. And they're also perfectly capable of learning and understanding other languages, even if they don't hold the capabilities to speak them. But the Empire looked past all this, simply viewing them as animals they could exploit.
After the creation of the Empire, Kashyyyk was invaded and most Wookiees were taken into slavery. They made for the perfect workforce because of their incredible strength and endurance, and they weren't strong enough in number to take on the entirety of the Empire. But their slavery wasn't just limited to their home planet, as Wookiees were shipped to places like the spice mines of Kessel and the Death Star to work the construction. And as seen with Chewbacca in Solo: A Star Wars Story, some were even kept as pets to torture.
However, even though it took a long while, some Wookiees were able to revolt thanks to the Rebel Alliance. Uprisings aided by rebels like Saw Gerrera took place all across the galaxy, with the main crew of Star Wars: Rebels even starting off the show by rescuing Wookiees from a slave transport. And after the destruction of the second Death Star, Han Solo and Chewbacca led an attack to liberate the Wookiee homeworld. Their plan was successful, and not long after, the Empire was gone for good.
Still, it took far too long for the Wookiees to earn respect. Famously, Princess Leia called Chewbacca a "walking carpet" after first meeting him -- a prime example of the discrimination they often faced. And unfortunately, their population greatly decreased during the Empire's reign, as many didn't survive slavery and were killed for sport. But, despite their rough exterior, Wookiees have proven to be one of the best species in all of Star Wars by helping save the galaxy on more than one occasion.