Why did Bernie Sanders win a narrow victory in Michigan, when polls showed Hillary Clinton with a huge lead? Nobody really knows, but there’s a lot of speculation that Mr. Sanders may have gained traction by hammering on the evils of trade agreements. Meanwhile, Donald Trump, while directing most of his fire against immigrants, has also been bashing the supposedly unfair trading practices of China and other nations.

So, has the protectionist moment finally arrived? Maybe, maybe not: There are other possible explanations for Michigan, and free-traders have repeatedly cried wolf about protectionist waves that never materialized. Still, this time could be different. And if protectionism really is becoming an important political force, how should reasonable people — economists and others — respond?

To make sense of the debate over trade, there are three things you need to know.

The first is that we have gotten to where we are — a largely free-trade world — through a generations-long process of international diplomacy, going all the way back to F.D.R. This process combines a series of quid pro quos — I’ll open my markets if you open yours — with rules to prevent backsliding.

The second is that protectionists almost always exaggerate the adverse effects of trade liberalization. Globalization is only one of several factors behind rising income inequality, and trade agreements are, in turn, only one factor in globalization. Trade deficits have been an important cause of the decline in U.S. manufacturing employment since 2000, but that decline began much earlier. And even our trade deficits are mainly a result of factors other than trade policy, like a strong dollar buoyed by global capital looking for a safe haven.