The Sakura Kingdom (literally meaning "Cherry Blossom Kingdom"), formerly the Drum Kingdom (ドラム王国,Doramu Ōkoku?), is a kingdom located on Drum Island. This is where the Straw Hats'doctor (Chopper) came from.
When the Sakura Kingdom was the Drum Kingdom, it was ruled by the royal family of Drum Island. It once had a kind ruler, and many of the greatest doctors on the Grand Line. But when the king died, and his son (Wapol) came into power, things took a turn for the worse. Wapol issued that the twenty best doctors (the Isshi-20) work exclusively for him, while the rest of the doctors were banished or killed.
Then, the Blackbeard Pirates came and invaded the island, and Wapol fled in fear. Since then, Drum Island was left without a ruling government, and Dr. Kureha took residence in his castle. When Wapol came back to try to retake his kingdom, it was also the same time when the Straw Hat Pirates landed on the island, looking for a doctor to help sick Nami. Wapol then tried to destroy Dr. Hiluluk's flag which was standing on top of Drum Castle. In response to this, Luffy used Gomu Gomu no Bazooka to knock Wapol out of the kingdom.
After Wapol was blown away by Luffy, the people of the kingdom decided to make Dalton the king, who decided to erase the bitter effect of Wapol's actions by renaming the kingdom "Sakura" in memory of Dr. Hiluluk and his goal of using the sakura to cure all diseases of the heart. The new kingdom also uses Dr. Hiluluk's Jolly Roger as the flag.