Tariff
A tariff is called an optimal tariff if it's set to maximize the welfare of the country imposing the tariff. It is a tariff derived by the intersection between the "trade indifference curve" of that country and the offer curve of another country. In this case, the welfare of the other country grows worse simultaneously, thus the policy is a kind of "beggar thy neighbor policy". If the offer curve of the other country is a line through the origin point, the original country is in the "condition of a small country", so any tariff worsens the welfare of the original country.
Trump tariffs
On March 22, 2018, Trump signed a memorandum under the Section 301 of the Trade Act of 1974, instructing the United States Trade Representative (USTR) to apply tariffs of $50 billion on Chinese goods. Trump stated that the tariffs would be imposed due to Chinese theft of U.S intellectual property. Trump said that his planned tariffs on Chinese imports would make the United States "a much stronger, much richer nation." However, the steps toward imposing the tariffs led to increased concerns of a global trade war.