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Applications regarding non-state combatants

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.

An opposing theory holds that, in response to extremist ideology and unjust governments, an invasion can change the government and reeducate the people, making prolonged resistance unlikely and averting future violence. This theory acknowledges that these changes may take time—generations, in some cases—but holds that immediate benefits may still be won by reducing membership in, and choking the supply lines of, these covert cells. Proponents of the invasion strategy in such conflicts maintain the belief that a strong occupying force can still succeed in its goals on a tactical level, building upon numerous small victories, similar to a war of attrition.

Contemporary debate on this issue is still fresh; neither side can claim to know for certain which strategies will ultimately be effective in defeating non-state combatants. Opponents of the invasion strategy point to a lack of examples in which occupying or peacekeeping forces have met with conclusive success. They also cite continuing conflicts such as Northern Ireland, Israel, Chechnya, and Iraq, as well as examples which they claim ultimately proved to be failures, such as Lebanon, and Afghanistan. Supporters of the invasion strategy hold that it is too soon to call those situations failures, and that patience is needed to see the plan through. Some say that the invasions themselves have, in fact, been successful, but that political opponents and the international media skew the facts for sensationalism or political gain.