Why did Taizong’s final years include expensive preparations for another Korean expedition?

Taizong’s expensive preparations for a final expedition against Koguryŏ during his final years were driven by his desire to address a significant failure in his otherwise successful foreign policy.12

The conflict began in 642, when an internal coup in the northern Korean state of Koguryŏ led its new ruler to attack Silla, a Tang vassal state. Despite the advice of most of his ministers, Taizong initiated an invasion of Koguryŏ in 645. The Tang forces, supported by allies in Manchuria and southern Korea, were ultimately forced to withdraw after suffering heavy losses. Following a second inconclusive campaign in 647, Taizong spent the remainder of his reign building a vast fleet and dedicating significant resources to these costly preparations in an effort to achieve a decisive victory.12