Guerrilla warfare is an increasingly common form of warfare in the modern era.

Due to the invention of nuclear weapons, the face of war changed dramatically after World War II. Because of the destructive potential of these new weapons, it was imperative that war did not escalate to a point where nuclear weapons became involved. This split warfare into 3 categories: nuclear warfare, conventional warfare, and guerrilla warfare.

While conventional warfare is usually between two different states, guerrilla warfare is more commonly intra-state conflicts associated with civil war. Guerrillas most commonly are fighting against the established forces of the government, and use hit and run tactics, unlike the army under the government's control, which will more likely use conventional tactics. The chief advantages of guerilla warfare are mobility and elusiveness. Since you never field any solid military units, the standing army has nothing to direct itself against.

Guerrilla warfare is dependant upon the common people for it's supplies of food, money, and recruits, and is typically only successful if they have popular support. While they may have their arms supplied by a third party nation, and even heavy weapons and monetary aid, the third party usually does not contribute troops.

Examples of modern day guerrilla wars:
Vietnam War
The Cuban Revolution