Although Sima Yi was so infuriated by the family letter that he felt like his lungs would explode, his mind was unaffected. On the contrary, his thoughts were extremely clear—the Southern Huns were doomed.
The Emperor issued a decree; failing to comply is nothing but handing a weapon to the person behind the scenes. Merely ignoring the Emperor's majesty is justification enough for destroying the Southern Huns.
Among the ten unforgivable crimes of ancient times, rebellion, disrespect, and filial impiety are included. Just one of these crimes spells doom for the Southern Huns. Not heading south upon the Emperor's decree can at most affect Liu Xie's reputation.
However, to speak truthfully, if Liu Xie issued the decree summoning the Southern Huns to Chang'an and they did not arrive, it would indeed be a heavy blow to Liu Xie's prestige, not as dreadful as the other, but definitely significant.
