There are two longstanding narratives in economics about trade.
1. Trade is about comparative advantage. Two people or communities that specialize in what they can do best (relative to what they would be doing otherwise) and trade with each other are better off (enjoy higher levels of consumption) than they would be without trade. The story becomes more dramatic when you bring in specializations within communities: those who specialize in goods that have a comparative disadvantage (e.g. electronics assembly in the US) lose out when trade is opened up, although their losses are not as great as the gains experienced by the rest of the community. The lesson is clear: there are special interests who will try to impede trade liberalization, but they should be resisted (or bought off) by the rest of us. Economists, who know the score, have a particular obligation to expose and combat protectionism.
2. Trade, while essential for growth and development, is a potentially destabilizing aspect of national and global macroeconomics. The gold standard was doomed to fail, for instance, because the specie flow mechanism (outflows of gold resulting in reduced money supply and lower prices) did not, in practice, cause trade deficits to diminish over time. In the absence of other adjustment mechanisms, countries with persistent deficits were forced to adopt punishing austerity measures, and this imparted a contractionary bias to the global macroeconomy. In the current period of flexible exchange rates, it is still the case that unbalanced trade is not only the norm, but has even increased in scale and destabilizing potential. Deficit countries are particularly subject to financial crises, due to the accumulation of private and public debt. (This is the message of the fundamental macroeconomic identity.) On the national level, persistent trade deficits result either in chronic employment problems or addiction to Keynesian demand stimulus that should, in principle, be only temporary medicine. In a world of sovereign states, we are still far from establishing a financial architecture that can contain the destabilizing effects of capital mobility and unbalanced trade.
What is interesting is that there is almost no communication at all between these two narratives. In particular, the second invalidates the first: if trade does not balance at the margin (changes in imports exactly offset changes in exports), the microeconomic case for trade liberalization collapses.