Also paralleling Rome, Homer-Dixon predicts that Western societies’ collapse will be preceded by a retraction of people and resources back to their core homelands. As poorer nations continue to disintegrate amid conflicts and natural disasters, enormous waves of migrants will stream out of failing regions, seeking refuge in more stable states. Western societies will respond with restrictions and even bans on immigration; multi-billion dollar walls and border-patrolling drones and troops; heightened security on who and what gets in; and more authoritarian, populist styles of governing. “It’s almost an immunological attempt by countries to sustain a periphery and push pressure back,” Homer-Dixon says.

Meanwhile, a widening gap between rich and poor within those already vulnerable Western nations will push society toward further instability from the inside. “By 2050, the US and UK will have evolved into two-class societies where a small elite lives a good life and there is declining well-being for the majority,” Randers says. “What will collapse is equity.”

Whether in the US, UK or elsewhere, the more dissatisfied and afraid people become, Homer-Dixon says, the more of a tendency they have to cling to their in-group identity – whether religious, racial or national. Denial, including of the emerging prospect of societal collapse itself, will be widespread, as will rejection of evidence-based fact. If people admit that problems exist at all, they will assign blame for those problems to everyone outside of their in-group, building up resentment. “You’re setting up the psychological and social prerequisites for mass violence,” Homer-Dixon says. When localised violence finally does break out, or another country or group decides to invade, collapse will be difficult to avoid.

Europe, with its close proximity to Africa, its land bridge to the Middle East and its neighbourly status with more politically volatile nations to the East, will feel these pressures first. The US will likely hold out longer, surrounded as it is by ocean buffers.