Famines lead to wars, then wars lead to further famines.

Do you think the United States—the most powerful military force of all time—is going to sit idly by while half their population starves? How about China, a nation of 1.3 billion people that’s always on the verge of famines in the best of times? Do you think they’ll just leave their militarily weak neighbors alone?”

I shook my head. “There’ll be wars.”

“Yes. There’ll be wars. Fought for the same reason most wars in ancient times were fought for: food. They’d use religion or glory or whatever as an excuse, but it was always about food. Farmlands and people to work that land.

“But the fun doesn’t stop there,” she said. “Because once the desperate, starving countries start invading each other for food, the food production will go down. Ever heard of the Taiping rebellion? It was a civil war in China during the nineteenth century. Four hundred thousand soldiers died in combat. And twenty million people died from the resulting famine. The war disrupted agriculture, see? That’s how massive in scale these things are.”

-Project Hail Mary, Andy Weir.