One of the acceptable definitions of "Nation" is: A people who share common customs, origins, history, and frequently language; a nationality. We have not been a nation since the 60's. America was a creation of men who disagreed with and decried the concept of a static populous that was the norm in England and most European Countries in the 1700's. We were different. Our attitude about government and personal freedom for the first 200 years remained markedly (and demonstrably) different from the rest of the world.
That changed with un-checked legal and illegal immigration. To be sure, in the earlier years there was no check on immigration...but the hardships of existing and making your way in early America culled the herd, so to speak, and only those who thought they could survive the hardships (or had no choice) came. And those who failed, died.
But assimilation into a culture isn't a natural thing for most. If immigration is limited, those who come to America have no choice but to assimilate...they have no significant group oriented to "the old country" to support separateness. Sure, the Irish, Italians and Scandinavians and others had small enclaves, but that was mostly driving by being poor...as they gained success in business and employment, they voluntarily and happily left the enclaves and joined main-stream America.
In addition, the mores of the times forced assimilation on those who came to America...unabashedly. The courts had not yet decided that you could re-create your "old country" here in America. If you came here, it was to "join", not infiltrate and expand your previous existence
The Courts and unchecked invasion have changed that. The American Nation State no longer exists. As the article says, we are divided. And that removes the concept of "nation." We are now just a "country." And we have lost the common bond that used to exist. Almost half of our citizens want us to emulate European Democratic-Socialism; almost half of our people believe that a New World Order should rise to do away with America and other country identities in favor of a World Government.
When the Roman Empire collapsed, something else would rise. Today, that is no longer true. The America the founding fathers envisioned has disappeared...forever..