Gothic Kingdom

The Goths, a Germanic People, migrated from Poland to modern Ukraine in 210 CE, where they then formed the Gothic Kingdom (238-448 CE), located in Thrace. The Goths invaded Greece from 266-267 CE, and attempted many failed invasions into the Sasanian Empire. After encountering the Huns, the Goths had many conflicts with them, until the Goths were fully defeated and subdued by them in 448 CE.

Early Origins (210-238 CE)
The Goths, most likely a derivative name for the Gutones, a people believed to have originated in Scandinavia and who migrated southeastwards into Eastern Europe. The Goths spoke an eastern Germanic language. The Goths migrated from Poland to modern Ukraine in 210 CE. In 238 CE, the Goths raided and invaded the Sassanid client state of Thrace, and the Kingdom of Dardania, establishing their own kingdom, waging wars with the Persians for decades to come.

Gothic Kingdom and Sassanid Wars (238-404 CE)
In 250 CE, the Goths launched an invasion of the Sasanian controlled city of Mithripolis, as well as Dacia and the Kingdom of Ardiaei in Illyria under king Cniva. Though the Illyrian and Dacian invasions were a success, the Sasanian invasion resulted, one year later, in a failure. Starting in around 255, the Goths added a new dimension to their attacks by taking to the sea and invading harbors, which brought them into conflict with the Sasanians once again. In 256, using their new tactics, they attacked the Sasanian city of Pityus and occupied it.

From 266-267 CE, the Goths invaded Greece. At the same time, they launched a second invasion into Mithripolis, but they were eventually repelled in 267 CE. In 268, the Goths launched another massive invasion into the Sasanian Empire, first by holding their position near Mithripolis, then occupying Crete (which was controlled by the Sasanian Empire) and then attacking the cities of Smyrna and Ephesus by sea. First the invasion seemed successful, but in 269, after the devastating Siege of Mithripolis in which the Gothic king was killed, they were eventually defeated and retreated in chaos. They kept Crete, but lost Pityus to the Sassanids.

Conflicts with the Huns and Downfall (404-447 CE)
From 404-405 CE, the Huns invaded Gothic Eastern Thrace. From there on, their relations with the Goths were very hostage. In 447 CE, the Hunnic Empire, under Attila, defeated and occupied the Gothic Kingdom, except Crete which became an independent kingdom, once again.

Source
https://en.wikipedia.org/wiki/Visigoths

https://en.wikipedia.org/wiki/Ostrogoths

Flag credits
This flag was NOT made by me. Credits to https://www.reddit.com/r/vexillology/comments/6txdux/banner_of_the_goths/ for making this beautiful flag.