In today's globalized economy, all countries rely on foreign resources. These include not just physical raw materials, but also manufactured products, services, and knowledge.
For example, the United States currently imports 20 percent of its food supply. That overall figure breaks down, though, to importing 75 percent of seafood and 35 percent of fresh produce. Often supplies are seasonally adjusted; for example, imports contribute to the ability to eat healthy fresh foods in winter. On the other hand, the United States is a net exporter of corn and soy beans.
In petroleum products, the United States is a net importer, with about 25 percent of supplies imported. However, quite importantly, with the advent of fracking, it has become a "swing producer," producing more when global prices are high (and thus reducing imports) and importing more more prices are low.
In many consumer goods, such as clothing and electronics, the United States is a net importer from lower-wage and more efficient countries that can produce goods at lower prices. An end to imports would mean large rises in consumer prices.
Many key materials such as rare earths and natural industrial diamonds are not normally found in the United States, and thus the United States relies on them.
Many companies outsource services abroad and would require massive restructuring to bring such jobs back; even worse, the higher costs of U.S. labor would result in collapse of many firms under such a scenario.