Immigration made America great. First, no one has proved America would not have achieved greatness with no immigration after 1776. It gave us the political corruption and proletariat we had avoided like the plague. Third, mass immigration was a response to the specific circumstances of a country needing vast amounts of unskilled labor for agriculture and industrialization. Land grants also made America great.
Immigration is America's tradition.
First, our traditional level of immigration, averaged year by year, is something like the equivalent of 200,000 per year, not the million-plus we get today. Second, immigration was not a tradition, it was just a fact. A tradition is something that is honored and valued as being part of the heritage that makes us the nation we are. Immigration, eo ipso , corrodes our national identity and must be ameliorated by other factors, like assimilation, that preserve it. Third, our historical immigration was, to be quite blunt, European.