Complex sentence: With Zaifeng now firmly tied to her, Cixi no longer viewed him as a threat, and when Zaifeng and Youlan's first son Puyi was born in 1906, Puyi became a likely heir to the throne.
Split into simpler sentences: With Zaifeng now firmly tied to her, Cixi no longer viewed him as a threat. When Zaifeng and Youlan's first son, Puyi, was born in 1906, Puyi became a likely heir to the throne.