Jobs follow technologies. When one is in an obsolete industry (cannot compete internationally) there will be pain.
Theory can tell you why the pain exists. Capitalism is a Storm of Created Destruction according to the most conservative of economists, Schumpeter). Life can get very hard when you are on the wrong side of the economic changes.
As others have pointed out we have not had true Free Trade, perhaps ever.
If a Free Trade Agreement is 20,000 pages, it ain’t Free Trade.
Creative destruction is good inside the USA. Americans can move to a new state where a new factory has been constructed. Once off shored there is no job.