Gonna state the obvious: freedom and peace. People mention money but money followed technological boom. And, yes, peace derived from military.
You might clarify "domestic peace". America has been one of the most secure nations in history from large-scale domestic invasion (it's essentially never happened: Pearl Harbor, isolated terrorist attacks, and "open borders" don't come close). That said, it has virtually always been actively involved in foreign conflicts and shadow wars during its 250 year history.
And yes, it's domestic security that enables long-term investment in science.
I would clarify it as relative peace. People simply left other parts of the world to pursue their dreams. If Europe weren’t basically war torn every couple of decades all the way up to the end of WWII, America might not have made it this far. And that’s why I don’t believe China will ever be that great until they reject pseudo-communist regime.