The new powers are contested among the nobility and royalty are habitually servants. With the removal of political foes, making military force is very much down to strength degradation.
A new scramble for power among officials, nobles and royalty, pretty consistently. To eliminate political enemies Thus undermining the military strength to many.
A new nobles usurped among officials and members of the royal family from time to time. The removal of a political enemy. The troops were undermined the strength down a lot.