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 in United States history
The Canadian-American Reciprocity Treaty increased trade between 1855 and its ending in 1866. When it ended Canada turned to tariffs. The National Policy was a Canadian economic program introduced by John A. Macdonald's Conservative Party in 1879 after it returned to power. It had been an official policy, however, since 1876. It was based on high tariffs to protect Canada's manufacturing industry. Macdonald campaigned on the policy in the 1878 election, and handily beat the Liberal Party, which supported free trade.