The internet has created enormous global changes. People can interact with others anyplace, anytime almost instantly. Much of the infrastructure including energy, healthcare, and telecommunications is interconnected globally. The world has added another domain to international conflicts: Cyber War. In this paper, we explore legal principles which would be involved in a cyber attack. Existing laws of warfare applied in the physical realm do not translate equally as well in the cyber domain. Due to the difficulty of attribution, the invisibility of borders and the need to react quickly, we may need to develop legal principles that allow immediate and appropriate responses. This is an area that needs very careful and thoughtful review