I had a professor in an intro econ class that said: "If a free trade agreement was actually about free trade, it would be 1 page, have some political fluff about cooperation, and 1 sentence saying that the two or more parties agree to free and unencumbered trade. Instead NAFTA is X hundred pages long." The problem in recent history was almost never geo-politically focused unlike the more recent trade events. Most usually trade agreements are dictated by lobbies and political associations (Farmers, Manufacturers, etc.) because they know that, contrary to popular opinion, there are ALWAYS losers in any trade deal, and those losers can be very detrimental to the careers of policymakers in Washington.
Most people are taught that free trade in every shape, form, and fashion is good. In layman's term that is true. The economic welfare of the country engaging in trade is increased, but the gains are distributed unevenly and are considered in the aggregate. Typically, these gains are the sum of wins and losses in every industry as the economy adjusts to new prices and competition. Think of it this way: a rising tide lifts all boats, but they don't tell you that the tide is preceded by a storm or even a hurricane that destroys or severely injures the older, less-maintained boats. The truth is free trade forces the less competitive firm out of business. People may complain about the supposed death of American manufacturing, but no one stops buying shit made in China. Then, businesses try to find ways to cut cost, and the highest cost of nearly any business is labor so they start layoffs. Then, they realize that production is down, and bring in cheaper labor often younger and less experienced, or in some industries immigrants willing to work for less, hence the "THEY'RE TAKING ER JERBS" mantra. People need someone to blame for their misfortune and inability to change with the times, and that blame often falls on foreigners, greedy companies, or welfare recipients, and sometimes technology (which is always seen as a necessary evil and people relent), but never free trade.
In fact, there is a fascinating article that I read in college, incredibly dense and lengthy but worth the read, concerning unchecked, accelerated globalized trade as the major cause of tension from late 1800s to 1990's that can be used to draw parallels to the high levels of nationalism and polarization in society experienced over the last decade aka Brexit, 2016 election, etc.
BLUF: The reason for tariffs is typically lobbies because no one wants to compete with foreign competition and risk sending your economic welfare to some other industry. And seriously this book is eye-opening and illuminates the economic machinations of our competitors along with the naivete in Washington: War by Other Means: Geoeconomics and Statecraft
