Serious answer: With the climate crisis some areas in the world will become impossible to live in (look up wet bulb temperature). Now you have a massive amount of refugees who want to go somewhere else, if necessary by force.
When there's suddenly millions of people pushing against country borders all it takes is a few shots and you have a war.
Not just high wet bulb temperatures, but also severe disruption to fresh water and shifting rainfall pattern. What used to be consistent patterns in nature that prevented droughts might change due to the changing climate. An example of this is mountains becoming ice-free, making them stop releasing fresh water throughout the year as the ice melts.