The US encouraged Panama, a province of Columbia, to rebell and start a revolution. With the help of the US Navy, they won their country's independence. In gratitude, they gave the US a ten mile wide zone to build a canal. American engineers built the canal for the next decade, but they had to deal with disease. The effort to control yellow fever, malaria, and bubonic plague succeeded, but thousands of workers had already died.
As Mexicans began to protest Porfirio Diaz's harsh rule, political parties opposed to Diaz rose. Strong leaders rose from different parts of Mexico, such as Francisco "Pancho" Villa and Emiliano Zapata. These and other armed revolutionaries won important victories against Diaz's army. By 1911, Diaz stepped down and new elections were called for.
The US fought in the Cuban war for independence, which became known as the Spanish-American War, for two reasons. The first was that the US had substantial business holdings in Cuba. In addition, the Spanish forced many Cuban civilians into concentration camps, which the US objected to. US forces first attacked the Philippine Islands, another Spanish colony. Unprepared for a war on two fronts, the Spanish military quickly collapsed. With its defeat, Spain handed over Puerto Rico, Guam, and the Philippines to the US.
At the order of President Fillmore, Commodore Matthew Perry sailed into Tokyo Harbor with four ships. The Japanese were amazed by the gigantic steamships and cannons. The Tokugawa shogun had no choice but the receive Commodore Perry and the letter he brought from President Fillmore. The letter requested free trade between the two countries. Commodore Perry was supposed to return in a year with a larger fleet to receive Japan's reply. The reply was the Treaty of Kanagawa, in which Japan promised to open two ports at which US ships could take on supplies.
China was extremely self sufficient and had little need to trade with Europeans. The British smuggled opium, an addictive narcotic, into China. As more and more people became hooked on opium, the British pushed more of the drug into China. Eventually war broke out-- the Opium War. Battles took place mostly at sea. The Chinese lost due to their outdated ships versus Britain's steam powered gunboats. In 1842, the Chinese signed the Treaty of Nanjing, which gave Britain the island of Hong Kong for 99 years. Another treaty, signed in 1844, granted US and other foreign citizens extraterritorial rights.
The US feared that European countries would try to reconquer new republics in Latin America. President Monroe issued to the Monroe Doctrine which stated that "the American continents...are henceforth not to be considered as subjects for future colonization by any European powers." The US did little to enfore the doctrine until the Spanish-American War.