Complex sentence: Then Mithridates VI gave Socrates command of a Pontic army, with which he invaded Bithynia, with the support of Mithridates and his army, eventually approaching the capital, Nicomedia, where Nicomedes IV barricaded himself inside his castle.
Split into simpler sentences: Then Mithridates VI gave Socrates command of a Pontic army and he invaded Bithynia with the support of Mithridates and his army. As Socrates marched across the countryside of Bithynia, he eventually approached the capital, Nicomedia, where Nicomedes IV barricaded himself inside his castle.