Complex sentence: The campaign was unable to be put in action, since the Manchus assimilated the massive Chinese army into their own and the Korean military, although reformed and expanded, was no match against combined Manchu and Chinese forces.
Split into simpler sentences: The campaign was unable to be put in action, since the Manchus assimilated the massive Chinese army into their own. The Joseon military, although reformed and expanded, was no match against the combined Manchu and Chinese forces.