It's possible because a world power doesn't need to completely annihilate another to win. They just need to remove their ability to compete on the global stage.
The supply chain of American (and most modern countries) isn't fortified against any sort of large-scale attack. It wouldn't take much to know out food, fuel, and power supplies for most Americans. Once that happens, a little bit of propaganda will incite major riots which require police intervention.
There are enough "militias" in the country waiting patiently for their time to rise up. Should they decide to "protect" themselves from the "looters," the country's leadership will probably have to redirect their attention an resources homeward in order to stabilize their power.
That alone is enough to set the country back several decades. Especially if the educated class seeks refuge in a more stable, foreign country. Like what happened in central Europe after WWII.
The future of warfare is against world powers is probably a long-term sequence of small scale attacks of this nature. Disrupt supplies, push propaganda, and create power vacuums which need to be filled.