Sugar plantations were the backbone of the European American Hawaiian economy. Sugar was also the main Hawaiian export to the United States. For many years, Hawaiian sugar was sold tariff-free in the United States. In 1890, the US Congress passed the McKinley Tariff Act Tariff Treaty of Nanjing Tariff Treaty of Kanagawa , which removed tariffs on all foreign sugar imports. This change meant that Hawaiian sugar producers had to compete with other nations in the US market.