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.
Tariff
The overall change in welfare = Change in Consumer Surplus + Change in Producer Surplus + Change in Government Revenue = (-A-B-C-D) + A + C = -B-D. The final state after imposition of the tariff is indicated in the second diagram, with overall welfare reduced by the areas labeled "societal losses", which correspond to areas B and D in the first diagram. The losses to domestic consumers are greater than the combined benefits to domestic producers and government.