Answer:
Expansionism was the most important issue prevailing during the era of the Manifest Destiny. There was a decisive question of expansion of slavery. Whether there'll be slavery in the new areas that were settled.
The northerners and the southerners defined the Manifest Destiny in different ways. The Manifest Destiny is concerned with the increase of capitalism and democracy around the nation. Hence Expansion of slavery was the issue during Manifest Destiny era.