the United States is not now and may never be as open to trade as the United Kingdom has been since the reign of Queen Victoria.
This view is widely held. Indeed I held this view until a few days ago. Its falsity would not undercut anything substantive in Krugman's case for free trade, but it is false. The place to see why is in this article by George Mason University economist John Nye. Check out his eye-popping graph of the average tariff rates for Britain and allegedly protectionist France. The whole article is worth reading. Here's his book on the topic.