The Boii Union was a union of Boii tribes south of the Po River.
At its founding, the Boii Union was very small. However, it had expanded to include everything, that was formally Boii, South of the Po River except for the West Boii Union.
Conquest of the West Boii
The Boii Union Wanted all Boii land south of the Po. To achieve this, the Boii invaded the West Boii. After many battles, the West Boii fell to the Boii in 370 BC. By 365 BC, the last of the independent Boii south of the Po were conquered.
Formation of the Cisalpine Empire
In 240 BC, the Boii Union began a long decline. Many tribes declared independence and the Union was on the verge of collapse. Finally in 90 BC, the Boii Union united with the Senone Empire to form the Cisalpine Empire, ending the state.
The Boii Union was a confederation. This meant that many rulers had powers in their respective state. There was, however, an emperor but he had no power outside of foreign affairs.