Chen Mo did not break his promise. When she initially agreed with Han Jingyun, she merely promised to send troops to suppress the Ning clan, but never said she wouldn't interfere in the affairs of the Wan Kingdom.
Moreover, after gaining control over the Wan Kingdom's military, he wouldn't oppress the royal family like the Ning clan did. As long as the royal family dutifully paid the annual tribute, Chen Mo would not meddle with its internal affairs.
Of course, to be strict, what Chen Mo did was somewhat unscrupulous. The princess had already given herself to him entirely and even sacrificed a great deal of gold and silver.
But from a national interest standpoint, Chen Mo's actions were entirely reasonable.
The Wan Kingdom was like a docile sheep. When Great Song began to decline, the Wan Kingdom started to act duplicitously toward Great Song, ceasing to pay the annual tribute. Although they never invaded Great Song, they were quite restless.
