During a recent cybersecurity competition, teams of students conducting a mock exercise unintentionally caused the U.S. to start a (fake) war. The students were given a variety of options, including diplomatic ones, for responding to a cyberattack by China. The majority of them took an aggressive approach, known as “hack back,” with disastrous consequences. The mock exercise shows how tempting it is to launch a counterstrike in response to a cyberattack — and the potential for significant unintended consequences.