Trade

Why countries exchange at all.

Comparative advantage

Specialising where your opportunity cost is lowest, then trading, can leave both sides better off even when one produces everything more efficiently.

The aggregate gains are real and so are the concentrated losses.
Displaced workers bear costs that the average figure hides entirely.

Barriers

Tariffs and quotas protect specific industries at a cost spread across all consumers. That asymmetry, concentrated benefit and diffuse cost, explains much of trade politics.