Complex sentence: His mastery was soon contested by the Visconti of Milan, but not even their rule was undisputed, as Pandolfo III Malatesta in 1406 took possession of the city, but in 1416 bartered it to Filippo Maria Visconti, who in 1426 sold it to the Venetians.
Split into simpler sentences: His mastery was soon contested by the Visconti of Milan, but not even their rule was undisputed, as Pandolfo III Malatesta in 1406 took possession of the city. However, in 1416 he bartered it to Filippo Maria Visconti, who in 1426 sold it to the Venetians.