It's extreme economic shocks generally. As in sudden changes in economic conditions that lead to a rapid and pervasive decline in living conditions. It's clearly not any absolute level of poverty. The poverty level that will trigger war in Bangladesh is far lower than what would trigger war in the US. And a country at South Sudanese levels of poverty going to a slightly higher level of development (say Bangladeshi levels) wouldn't trigger conflict, even if they were still far worse off than levels that would trigger war in better off countries. But the absolute floor at which war (or widespread systemic violence and civil unrest at least) becomes almost inevitable is when people can no longer afford food. There have been a few studies that suggest a link between sudden increases in food prices and civil unrest and wars.

Yeah, you're right, that's probably the best description. Economic shocks or a handful of other shocks (such as forced enrollment in a risky endeavor, e.g. being drafted into the army).

Things got pretty bad during the Vietnam war. There is a reason why the draft was abolished.

Wasn't that because of the draft, though? That counts as "major" since you're being shipped off to be killed. But how likely is a new Vietnam? US involvement in wars doesn't require a draft anymore...

My point, inequality itself is rarely the trigger. It needs to pass a limit, usually threatening livelihood. And it needs to be super widespread (think 20%+ of the population).

Males 18-25 still have to register for the draft, there are just enough voulenteers currently

The hungry and homeless generally don't involve in political violence. Terrorists arise from well-to-do middle class.

Their are over 1 million homeless people in the US in the average year. The issue is more when people that could not conceive of being homeless start to be homeless that you get revolutions.