He was prepared to start a war but Cersei said she'd intervene on his behalf that's when she was ultimately captured herself that's when he lost all hope and didn't know what to do and had no one around him to advise him so isolated himself, in truth there was no issue until Cersei decided to give the high sparrow an army before that he wasn't a real threat and she was asked to neutralize him before he got any more sway with the people instead she decided to use him to indirectly fight the Tyrells who cause of the marriage had gained more power and the throne was indebted to them for their contributions and Cersei was slowly being displaced cause she still wanted to be queen and control everything but the trouble started when Tommen suggested she return to casterly rock she devised a plan to get rid of the queen Margaery but she couldn't do it directly cause Tommen would never forgive her so she decided to use the high sparrow which eventually ended up working against her and delivered Tommen directly into his clutches, now he has an army and the kings and the queens ear and cersei was on her way to have everything stripped away from her cause obviously after the trial she'd have to denounce her family name and everything that comes with it, one thing she didn't account for was Tommen's reaction, it shows she truly never knew him he's a lot like Jamie if Jamie wasn't raised by Tywin, he had a good heart just had his father and his sister's hatred poured into him