Germanic warlords is perhaps not the correct term to describe those who held political and military power during the early European middle ages. Most held property in fief from a higher lord or even a monarch, and were obligated to fight for that lord or monarch. They often held titles such as Duke, Count, or even Earl. At the time, monarchs relied on these lesser nobles to furnish him with troops in the event of war, and also to fight for him themselves.
The most important factor in the movement to centralized monarchy was the ability of counts, lords, etc. to extend the areas under their influence by means of attacking territories held by others; or held by nomadic peoples such as the Magyars or Lombards. A prime example of this is Otto of Saxony, who defeated the Magyars at the Battle of Lechfeld, and was crowned Holy Roman Emperor by Pope John XII. It should also be pointed out that the support of the church was important. Although many Kings and Emperors were excommunicated for disagreements with the Church, most managed to maintain good relationships with the Popes, and used this factor to extend their own influence.
In England and France the change in relationship between Lords and Retainers allowed monarchs to increase the power under their control. If a nobleman died childless, the monarch claimed the late lord's property for his own. This was particularly true of the Capetian kings of France who over three hundred years absorbed a great deal of power and with that power assumed the right to administer justice throughout their realm.
In addition, rather than demand service and rents in kind from retainers, kings began taxing them. With taxes paid in cash, kings were able to establish standing armies which allowed them to increase their power. With standing armies, they were no longer dependent upon retainers to fight for them or furnish troops; and were therefore able to impose their authority over these same former retainers.