Why did Kublai Khan renew war against the Southern Song dynasty in 1267?

In 1267, Kublai Khan resumed military operations against the Southern Song dynasty primarily because he was convinced that the regime in the south had to be dealt with decisively.1

Although Kublai had previously offered to establish peaceful coexistence, his diplomatic efforts were rebuffed by the Song court. Specifically, the Song government interned his ambassador, Hao Jing, at the border and treated him as a spy. The Song chancellor, Jia Sidao, viewed the Song position as strong enough to risk this affront and chose to ignore the peace offer, instead focusing on strengthening military preparations against the Mongols. This detention and mistreatment of his envoys ultimately convinced Kublai that a peaceful resolution was not possible, leading him to initiate a full-scale renewal of the war.231