1) Selling and arming these two countries with devastating weapons is NOT isolationism.
2) It is debatable whether China will even risk her economy to attack these two industrial and technological giants with huge armies. And if these two countries have nukes of their own, that will be another deterrent.
The presence of U.S. troops is the ONLY deterrent to China’s overtaking the South.
You can debate all you like.
China’s objectives are not in question, as they deal in millennia, not election cycles.