In 1429 Florence besieged Lucca to take revenge on the republic. After several days of siege, the republic of Lucca asked Duchy of Milan for help. The Duchy sent Francesco I Sforza, who overwhelmed the Florentines with his army and forced them to retreat. A few days after the retreat of the Florentines, the inhabitants of Lucca arrested Paolo Guinigi, the leader of the republic, because he was said to have dealt with the Florentines. Florence later paid Sforza to abandon Lucca and, in 1430, Lucca was besieged once again. During the siege the Florentines tried to stem the Serchio to flood Lucca, but due to some errors the Florentine camp flooded. Meanwhile the Lucchesi asked again for help from Filippo Maria Visconti, duke of Milan, who, once again, acted indirectly (according to a previous treaty Milan could not interfere in the affairs of Florence) asking the Republic of Genoa to help Lucca. Genoa, relying on an ancient alliance with Lucca, asked Florence not to disturb Lucca. When Florence refused, Genoa sent an army of 6,000 men led by Niccolò Piccinino who attacked the Florentines on the Serchio and, after a bloody battle, they were forced to withdraw because the Lucchesi had taken them behind leaving the city.