The Zman looks at when Rome’s fate was sealed:
The die was most likely cast when the Republic began to compromise its own rules for limiting and distributing power. The system they had created was a reflection of the tribal realities of the early republic. In order to keep any one family from gaining too much power, they systematically limited the time anyone served in office. The system also forced an apprenticeship on those who went into public life. This had the benefit of making public men buy into the system. Therefore they were willing to defend it.
That meant the system had a policing mechanism to sort out enemies before they could cause trouble. An ambitious young man could not skip any steps on his way up the ladder, so once he got up the ladder, he was not agreeing to any changes in the process. Defending the system was a way to defend one’s prerogatives, but also a way to defend the system from lunatics. Verpus Maximus may be smart and talented, but he was not only going to wait his turn, he was going to do all the jobs necessary to prove his worth.
This system started to break down with the rivalry of Sulla and Marius. Sulla was the first man to hold the office of consul twice. He also got away with marching an army on Rome itself, in order to defeat his rival, Marius. Both of these acts were supposed to be disqualifying, but exceptions were made for expediency. Sulla sided with the Senate so the Senate bent the rules to serve themselves. A good case can be made that this is the point when it was all over for the Republic.
It was just a matter of time before someone used Sulla as a precedent.
The die-casting point that sealed the fate of the USA is a little bit easier to determine, in my opinion. The 1965 Naturalization Act that eliminated the restrictions on immigration by national origin is the obvious one.
Notice that in both cases, it was abandoning tradition and loosening vital restrictions that proved to be the fatal act.