Complex sentence: Christian invaded at the head of a mercenary army of 20,000 men, but the Danish forces were severely beaten, and Christian IV had to sign an ignominious defeat, the first in a series of military setbacks to weaken his kingdom.
Split into simpler sentences: Christian invaded at the head of a mercenary army of 20,000 men. But the Danish forces were severely beaten, and Christian IV had to sign an ignominious defeat, the first in a series of military setbacks to weaken his kingdom.