Original: Rising from a lowly page at the court of the House of Stuart, he loyally served James, Duke of York, through the 1670s and early 1680s, earning military and political advancement through his courage and diplomatic skill.
Simplified: Rising from a lowly page at the court of the House of Stuart, he faithfully served the Duke of York through the 1670s and early 1680s. This gave him military and political advancement through his brave, clever skill.