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
With regard to independent third world countries or countries that did not have colony status in the 19th century (most of Latin America, China, Thailand, the entire Middle East), Western countries had exerted such pressure that most of them had signed treaties providing for the abolition of import duties. They were forced to open their markets to Western products, which allowed the massive entry of imported manufactured goods. Customs legislation could not provide for tariffs higher than 5% of the import value of the goods. Most of these "unequal treaties" were signed between 1810 and 1850, mainly at the initiative of the British.