Speaking from a liberal perspective, the threat of terrorism or an ethnic conflict can be vastly different depending on where you live. If you are a resident of France, you are much more likely staying awake at night worrying about terrorism. If you are a Rohingya Muslim in Myanmar, your primary fear is ethnic conflict. Of course, this question is posed as the largest global threat, and I would choose ethnic conflict. Terrorism is primarily driven by religious fundamentalism and will come to pass in time with religious reform; as well as decreasing poverty, increasing access to education and information to the poorest countries that currently serve as the incubators of zealots ready to die for a promised reward in the afterlife. Ethnic conflicts can rise from several different factors, the deepest of which is rooted in tribalism, etched into our subconscious from ancestral DNA. This can also be stoked by charismatic leaders seeking nationalism and outright, unchecked power in their respective nation.
As Nau states, a liberal perspective sees “…radical Islam as no worse than Christian fundamentalism and worry that Jewish fundamentalism is just as bad and provokes, through Orthodox demands for annexation of the West Bank and Gaza, much of the deep-seated Arab resentment. From this perspective, as the causal arrow suggests, it is better to focus on the roots of the poverty and despair that breed terrorism and downplay the ideological causes of the conflict” (2017, p.332). Sadly, with the way we are going, I could see a future on this planet where wars and genocides still occur, given the current slow burn reversal of civil rights and our (the USA) withdrawal from the UN Human Rights Council as of last week.
Nau, H. R. (2017). Perspectives on international relations: Power, institutions, and ideas (6th ed.). Thousand Oaks, CA: CQ Press.