Power and violence are not the same. Power is psychological, a moral force that makes people want to obey. Violence enforces obedience through physical coercion. Those who use violence may manage to temporarily impose their will, but their command is always tenuous because when the violence ends, or the threat of it lessens, there is even less incentive to obey the authorities. Control through violence requires constant vigilance. Too little violence is ineffective; too much violence generates revolt.
Violence is the weapon of choice for the impotent. Those who don't have much power often attempt to control or influence others by using violence. Violence rarely creates power. On the contrary, groups or individuals that use violence often find their actions diminish what little power they do have.
Groups that oppose governments often try to compensate for their perceived lack of power by using violence. Such violence simply reinforces state power. A terrorist that blows up a building or assassinates a politician gives government the excuse it wants to crack down on individual liberties and expand its sphere of influence.
When a government turns to violence, it is because it feels its power is slipping away. Governments that rule through violence are weak. Dictators have always had to rely on terror against their own populations to compensate for their powerlessness.
The U.S. would feel no need to fight wars in Latin America or the Persian Gulf if it had power in those regions. The only way to maintain control in the absence of power is through the continual use of violence. Protracted violence results in diminished power, making more violence necessary.