In the 20th and 21st centuries, questions arose regarding the effectiveness of the invasion strategy in neutralizing non-state combatants, a type of warfare sometimes referred to as "fourth generation warfare". In this case, one or more combatant groups are controlled not by a centralized state government but by independent leadership, and these groups may be made up of civilians, foreign agents, mercenaries, politicians, religious leaders, and members of the regular military. These groups act in smaller numbers, are not confined by borders, and do not necessarily depend on the direct support of the state. Groups such as these are not easily defeated by straightforward invasion, or even constant occupation; the country's regular army may be defeated, the government may be replaced, but asymmetric warfare on the part of these groups can be continued indefinitely. Because regular armed forces units do not have the flexibility and independence of small covert cells, many believe that the concept of a powerful occupying force actually creates a disadvantage.