^^Some economists point to 1971 as the year that the U.S. net debt was greater than our ability to pay it on time and therefore mark that year as when we effectively became a debtor nation for the first time since WWI. The causes were numerous, but the largest expenditures were for our military adventures abroad, which can be described as imprudent at best. Our involvement in the Vietnam War cost us approximately $1 million per minute for ten years (1967 dollars) beginning in January 1964 when we inserted an ever-increasing amount of Armed Forces regulars into the country, peaking at 250,000 in the summer of ‘68 (this figure does not include the massive Naval and Air support from offshore aircraft carriers and long-range support from Clark AFB and Subic Bay in the the Philippines, and the SAC B-52 bases in Guam, nor the black ops personnel operating in Thailand, Cambodia and Laos). This among other things, as mentioned by @zenvelo above, exacerbated an economy already beginning to feel the effects of growing European and Japanese exports into markets formerly dominated by the US.
Apparently, however, there are different ways of measuring whether or not we actually are a debtor nation and therefore there are arguments as to precisely when we became a debtor nation.
According to this New York Times Business Day article from 16 Sept. 1985, the US not only became a debtor nation for the first time the day before on 15 Sept. 1985, but went from the world’s greatest lender, to its greatest borrower:
“Massively reduced taxation under Reagan resulted proportionally in massively decreased Government revenue. This was just as most mainstream economists and common sense thinkers had predicted.
“To make things worse, to go along with this sharp reduction in the Government’s tax income, the Reagan administration simultaneously increased military spending—to levels the world had never seen. The combined reduction in Government income and increased expenditures overseas, and the consequent need for massive borrowing to make up the shortfall, transformed the U.S. virtually overnight from the world’s biggest net lender to the biggest net debtor.”
We are still a debtor nation today. This hasn’t changed. Based on this, I would say that, no, we do not enjoy a sounder economic foundation today than we did in the 1960s.