Complex sentence: After they had embarked their landing craft, the Swedes first torched some small boats on fire and pushed them out the river in order to obstruct the view for the allies, then at four o'clock in the morning of the 19th, the attack began.
Split into simpler sentences: After they had embarked their landing craft, the Swedes first torched some small boats on fire and pushed them out to the river in order to obstruct the view for the allies. And then at four o'clock in the morning of the 19th, the attack began.