D614G happened almost immediately. There were also likely mutations that made it better adapted to mammals/humans that we never saw before it became "good enough" to form a pandemic virus in humans.
Beyond that it was good enough with an R0 of >3.0 and ability to infect the entire world's population that it spread globally and faced no selection pressure to mutate. Once it was facing a population with a climbing amount of immunity it was forced to get better at infecting in order to go after the remaining fully susceptible population, and it faced selection pressure to become more immune evasive and to reinfect.
There are ongoing cyber attacks on US infrastructure every day, both by rogue groups and hostile state-sponsored actors. These attacks target key elements of US infrastructure, government agencies, NGOs and all of our major industries.
Short of an EMP that takes down most of the grid, I don't see any real danger from a cyber attack. The US has some of the best cyber security experts and it I find it unlikely that some foreign group could find some possible exploit to break into most systems and get past the teams in place to prevent that.
On the biological attack side, there is more danger as it may not be immediately obvious who released a pathogen. With effective quarantine measures, the scope of such a disaster can be controlled and thus, to win a World War 3, a hostile power would need to employ conventional means.
The US views biological weapons as WMD, and it is policy to respond in kind. Thus, things would go nuclear and we could look forward to mutual destruction, which anybody in their right mind would avoid. And regardless of the minority who are not in their right mind, the majority of people on earth would rather be alive than dead and so I doubt that there would be a kamikaze nation that decides to do something that foolish without regard for the possible consequences.
[1] - https://www.youtube.com/watch?v=biNxl7tiVSY [video][16 mins]