When your mind is trained in self-discipline, even if you are surrounded by hostile forces, your peace of mind will hardly be disturbed. On the other hand, if your mind is undisciplined, your mental peace and calm can easily be disrupted by your own negative thoughts and emotions. The real enemy is within, not outside. Usually we define our enemy as a person, an external agent, whom we believe is causing harm to us or to someone we hold dear. But such an enemy is dependent on many conditions and is impermanent. One moment, the person may act as an enemy; at yet another moment, he or she may become your best friend. This is a truth that we often experience in our own lives. But negative thoughts and emotions, the inner enemy, will always remain the enemy. They are your enemy today, they have been your enemy in the past, and they will remain your enemy in the future as long as they reside within your mind.This inner enemy is extremely dangerous. The destructive potential of an external enemy is limited when compared to that of its inner counterpart.... In a time when every country is a potential target for the nuclear weapons of others, human beings still continue to develop defense systems of greater and greater sophistication. I do not know if it will ever be possible to create a defense system capable of guaranteeing worldwide protection against all external forces of destruction. However, one thing is certain: as long as those destructive internal enemies of anger and hatred are left to themselves unchallenged, the threat of physical annihilation will always loom over us. In fact, the destructive power of an external enemy ultimately derives from the power of these internal forces. The inner enemy is the trigger that unleashes the destructive power of the external enemy.
Shantideva tells us that as long as these inner enemies remain secure within, there is great danger. Shantideva goes on to say that even if everyone in the world were to stand up against you as your enemies and harm you, as long as your own mind was disciplined and calm, they would not be able to disturb your peace. Yet a single instance of delusion arising in your mind has the power to disturb that peace and inner stability.
--from The Compassionate Life by Tenzin Gyatso, the Fourteenth Dalai Lama