Complex sentence: The advancing Norwegians quickly drove the Swedish rearguard at Gamleseteren against the main force, and Gahn quickly realized that there was a large force attacking the column from behind and threatened to encircle them.
Split into simpler sentences: The advancing Norwegians quickly drove the Swedish rearguard at Gamleseteren against the main force. Gahn realized that there was a large force attacking the column from behind, threatening to encircle them.