Heli Tiirmaa-Klaar wants to write the rules of cyber conflict. As Estonia’s first ambassador-at-large for cybersecurity, she’s at the forefront of a battle over what countries can and can’t do in cyberspace. Her ambition: to stop repressive governments from committing acts of war and aggression online, while ensuring they can’t use international law to suppress the free flow of information. “The challenge for the next five to 10 years is how to keep the global internet open, or whether it will be split,” she says.