Germa Kingdom (formerly): Sanji's former home country, whose family continues to reign as the kingdom's ruling family. It was formerly the center of a vast empire spanning the entire North Blue, but is now the world's only roaming, maritime kingdom. Currently, the Kingdom is assembling in the New World.
Sometime in the distant past, the entire of North Blue was conquered by the infamous Vinsmoke Family, thereby forming the Germa Empire (ジェルマ帝国,Jeruma Teikoku?); this reign was so cruel that it led to the Vinsmoke name becoming synonymous with evil. Since the empire's collapse (the cause of which is currently unknown), the family continues to rule over the maritime Germa Kingdom, and retains the considerable influence and status of royalty from their period of sovereignty.
One of the most famous stories from the North Blue is the tale of Mont Blanc Noland, an explorer from Lvneel. Over 400 years ago, Noland left to explore the Grand Line, traveling many places from Vira to Jaya. Upon his return home, he told everyone about his journey to Jaya and seeing of the City of Gold.
The king of Lvneel heard about the City of Gold and asked Noland to take him. Noland tricked the king into going to Jaya only to find the City of Gold was not there. Upon his return, the king sentenced Noland to death and he was executed, his last words being "That's it! The City of Gold must have sunk into the sea!".
This event was published, by the king, into a child's storybook, where parents would use it to teach their children not to lie, lest they end up like Mont Blanc Noland. People like Sanji, Bellamy and Sarkies, born in the North Blue, have came to know this story when reading it as children.
Note: This is a historic recording of the story (not the true event), the true story can be found here.
Conquest of Four Nations
Despite the loss of their former empire, the Vinsmoke Family's current patriarch maintains an ambition to reestablish the family's rule over the entire North Blue. This desire was a direct cause of the infamous incident known as the Conquest of Four Nations, in which the kings of four nations in the North Blue were assassinated by the Vinsmoke Family.
When citizens found out that the Donquixote Family was no longer under diplomatic immunity, they hunted down the family in revenge against all the ill-deeds done onto their loved ones by World Nobles, leading to Homing's wife to die from malnutrition that came from the deplorable shack they were forced to live in.
For two years, the remaining three family members were tortured and persecuted, suffering from poverty and starvation, until Doflamingo awakened his Haoshoku Haki and knocked out the commoners. This feat earned him the loyalty of a young gang—Vergo, Trebol, Diamante and Pica—who saw Doflamingo as their new king. The four would kill and destroy anything that displeased Doflamingo even the slightest, with many victims and towns suffering such fates.
Doflamingo eventually killed his father (which led Rosinante to run away), and after failing to reenter Mary Geoise, he returned to the North Blue, where he and his four friends built a new family to terrorize the quadrant of the sea before entering the New World.
16 years ago before the current time, the country known as Flevance was famous for its Amber Lead, a beautiful mineral that made the country to be wealthy, and is what earned the country the nickname "White City".
However, a century before this business boomed, the World Government and royal family of Flevance discovered that the mineral was poisonous when exposed to air, but chose not to reveal it, in order to maintain the profitable resources. This led to the poison to accumulate in the citizens' bodies, and each succeeding generation of offspring having shorter lifespans than their predecessors. It was 16 years ago that the full effect of this "Amber Lead Syndrome" came into visible effects, where the lead poisoning is showing on the victims' bodies.
Neighboring countries mistook this as a contagious disease, and quarantined the White City from the rest of the world, which led the country to retaliate, leading to its destruction, with Trafalgar D. Water Law as the sole survivor, who joined the Donquixote Pirates to take revenge on the world who shunned his homeland.
Although the World Government had allowed the royal family of Flevance to escape selfishly as the rest of the country fell, the world remained vigilant of the Amber Lead Syndrome, incorrectly believing that it was transmitted by human contact, shown when every doctor in the North Blue that Rosinante took Law to demonized the latter as a survivor who should be eradicated.
↑SBSOne Piece Manga — Vol. 13, Oda reveals where Usopp's goggles came from.
↑One Piece Manga — Vol. 83Chapter 832, The period of the Vinsmoke Family's conquest of the North Blue was described as being in the "distant past".