Hello. You didn't say which genocide you're referring to. However, genocides start because of ethnic, national, racial, religious and even political prejudices, the people involved in this practice, are authoritarian people and regardless of differences. To prevent the genocide, people can adopt measures that encourage the acceptance of differences between citizens, in addition to choosing their political representatives who do not encourage prejudiced and violent terms and behavior.
The international community can prevent genocides by paying attention to authoritarian, theocratic, ultra-nationalist governments that exhibit any type of harmful behavior. In addition, the international community can establish institutions that promote integration between people of races, ethnicities and religious. We cannot fail to mention that the international community must act in order to ban any hate speech proclaimed by leaders, in addition to harsh repressions for governments that incite genocide.