Complex sentence: In September the Schleswig-Holsteinian launched an attack at Mysunde, which was repulsed by the Danes and in October, the Danes managed to thwart the siege of Frederiksstad, but again failed to defeat the Schleswig-Holsteinians.
Split into simpler sentences: In September the Schleswig-Holsteinians launched an attack at Mysunde, which was repulsed by the Danes. In October, the Danes managed to thwart the siege of Frederiksstad, but again failed to defeat the Schleswig-Holsteinians.