Geography clearly has a big impact on the country's success.

Also the amount of riches (oil, gold, etc) the land happens to have.

The climate also makes a big difference. I believe there is a correlation between temperature and success, as in, countries with colder climates tend to do better. It might be because people simply die on the outside on the cold countries, whereas they can survive on the hot ones. And also that being too hot is just uncomfortable and makes you less productive.

But in the end what's most important is the people in the country. And as you mentioned, the US could potentially face the issue of an internal war.