I would suggest fear. Everyone who is afraid divides the world into "us" and "them." When afraid, we seek power, thinking it will make us safe. If we've been hurt, and are still afraid, and get power, we become angry and seek revenge. When this happens on a national level, the nation maintains power and attacks its perceived enemies. If those enemies are individuals or small groups who are themselves afraid, hurt, angry, and seeking revenge, then they are terrorists.
Terrorists use the power they have to terrify people, creating more fear. Governments use the power they have to terrify enemies, creating more fear.
From that point on, the cycle feeds itself.
One who chooses not to be afraid cannot be terrorized. One who chooses to love his or her neighbor and his or her enemy does not terrify others. When we all do this, we will share in the love that frees us from fear, and the greatest power of all, which is peace.