Complex sentence: After the decisive victory of the Viscontis at the Battle of Desio in 1277 it was occupied by Archbishop Ottone Visconti and the Marquis of Monferrato, Guglielmo (1278) and the following year the town was declared a possession of the mayor and the people of Milan.
Split into simpler sentences: After the decisive 1277 victory of the Viscontis at the Battle of Desio was occupied by Archbishop Ottone Visconti and the Marquis of Monferrato, Guglielmo (1278). The following year, the town was declared a possession of the mayor and the people of Milan.