Complex sentence: Podhorski agreed, then deciding that at first they would go to Stawy near Dęblin, the location of the main arsenal of Polish army, and then go to the Holy Cross Mountains and engage in guerrilla warfare.
Split into simpler sentences: Podhorski agreed, then decided that he would first go to Stawy near Dęblin, the location of the main arsenal of the Polish army. They would then move to the Holy Cross Mountains and engage in guerrilla warfare.