Original sentence: The Army leadership felt deeply betrayed by the King because they thought that while they had been negotiating in good faith he had duplicitously gone behind their backs in making  The Engagement with the Scots and encouraging a new civil war
Edited sentence: The Army leadership was deeply betrayed by the King because they thought that while they had been negotiating in good faith he had duplicitously gone behind their backs in making  The Engagement with the Scots and encouraging a new civil war