The United States was unquestionably founded as a white nation. And since the meaning of the term "white" is apparently contentious, we can define it as having ancestral origins in Northwestern Europe. In the case of the United States, we can further break down the precise meaning of "white" to entail those of English, Germanic and Scots-Irish ancestry. These were the major ethnic groups that immigrated to the New World in large numbers in the 17th century, who founded the United States in the 18th century, and who built a great nation in the 19th century.
Up until the Hart-Cellar Immigration Act of 1965, legislation in the U.S. had been specifically written to strictly limit non-white immigration into the country, beginning with the Naturalization Act of 1790, which applied only to "free white persons of good character". Later there was the Page Act of 1875, which was passed explicitly to prohibit "undesirable" immigration by non-whites (Chinese). And half a century later came the Immigration Act of 1924, which strictly limited the number of non-white immigrants into the U.S. (these quotas included Eastern Europeans and Slavs - lending further credence to our earlier definition of "white" Americans as applying to those of Northwestern European extraction).
The United States was founded and built by men of Northwestern European ancestry, and those men and their descendents made efforts over numerous generations to preserve the racial and ethnic makeup of the country they founded. The United States of America was never intended to be home to large numbers of non-whites. America was not founded as a melting pot, or a "nation of immigrants". It was, from the beginning and for the vast majority of its history, explicitly a nation comprised of citizens from Northwestern Europe. This is made very clear not only by the historical demographic data of the U.S., but by the surviving letters, publications and legislation of prominent Americans of the 18th and 19th century.
