Complex sentence: The Norwegian scorched earth policy, the guerrilla raid interdiction of supply chains by the residents of Båhuslen derived Charles of supplies and the Norwegian occupied fortresses behind his lines threatened his retreat if seriously weakened in combat.
Split into simpler sentences: The Norwegian scorched earth policy and guerrilla raid interdiction of supply chains by the residents of Båhuslen derived Charles of supplies. Further, the Norwegian fortresses behind his lines threatened his retreat if he became seriously weakened in combat.