[This is the beginning of a long piece by me on the reasons for limiting immigration, that will appear in Saturday's print edition of the Wall Street Journal.]The immigration debate in the U.S. has been contentious for decades, but Donald Trump’s candidacy and election have taken it to a new level of polarized animosity. Politicians and the public have focused, understandably, on Mr. Trump’s promise to build a “big, beautiful” wall on the U.S.-Mexico border and on what should be done with the millions of illegal immigrants currently in the country.These are certainly important issues. But they are enforcement issues. They are less fundamental than a question that too often goes unaddressed in our debates: Why limit immigration at all? Almost everyone at least pays lip service to the need for limits of some kind, but we don’t often enough challenge each other to explain what limits we support and why.