They did not have to contend with Nuclear Weapons. If their enemies had them and were suicidally hellbent on using them it would have been a pretty short run wouldn't it?
Will we leave a legacy? I think so.
Let's be serious, if we fall ... everyone falls with us. Who will be leaving a legacy to? Cockroaches?
Same enemies.
Welcome to the 1300-Years War. Al Qaeda speaks of us as "Romans" more often than as "Crusaders".
The cannons that bashed down the Walls of Theodosius were just as astonishing to the medievals as nuclear weapons are today. Of course the US had nuclear weapons 60 years ago, and has an arsenal of thousands of them. I submit that a group that uses one of their first 10 nuclear weapons has a pass to oblivion, not a reasonable approach to the destruction of the US.