Complex sentence: In 1403 Bellinzona came under the control of Alberto di Sacco of Val Mesolcina, who held it until 1419 before it was taken over by Uri and Obwalden, which expanded into the Leventina Valley.
Split into simpler sentences: In 1403 Bellinzona was taken under the control of Alberto di Sacco of Val Mesolcina. He held Bellinzona until 1419 when it was taken over by Uri and Obwalden which had expanded into the Leventina Valley.