This is a little like saying fire is hot, stones are hard, and that water is wet.
From within. It's been going on for decades, but has picked up steam over the last 30 years, accelerating lately.
Here's how it happened, and yes, it was planned:
1) Destroy the American education system by no longer teaching math, science, history, or critical thinking skills.
2) Destroy concept of rugged individualism by drugging male schoolchildren, and making life difficult for the self-employed (taxes, regulations, etc....)
3) Transfer wealth from the United States to other countries. This is done through direct foreign aid, the World Bank, and loan guarantees.
4) Transfer jobs from America to the third world. Start with manufacturing, and advance to R&D later.
5) Expand debt on all levels, empowering bankers and politicians.
6) Allow uneducated barbarians to immigrate to this country. Get them to vote and become an additional drain on our resources.
America will be over within a decade unless our disasterous course is promply reversed.