‘War’ as we have known and understood it until the 20th century – conflicts involving kinetic weapons such as bullets, missiles, machine guns, tanks, ships, aircraft carriers, submarines and fighter jets – is rapidly being influenced and complemented (but not replaced) by a new 21st-century cyberwar era involving Internet-based technologies. The UN Charter represents one of the primary sources of international law concerning the security of the international community, which urges states to refrain from the ‘use of force’. But does the UN Charter, specifically Article 51 (known as the ‘collective self-defense’ provision) provide justification for the ‘use of force’ in certain, limited circumstances, when it is necessary for a state to defend itself against certain cyberwar tactics? This article argues that it does, and second, suggests language to revise Article 51 of the UN Charter to reflect better the rise of cyberwar tactics by both state and nonstate actors in the 21st century.