Mexico

At the end of the 17th century, Spain became a declining empire with diminishing revenues and loss of both political and military influence -- especially against other rising powers such as Britain. The Bourbon Reforms (1747-1808) changed the colonial system in the Americas to that of an intendancy. These reforms were directed to increase tax revenues, but became very unpopular among the colonies as most intendants were peninsulares, or people born in Spain.

This power shift increased unrest among criollos, or people of Spanish ancestry who were born in the Americas, who ultimately attained the support of both Amerindians and mestizos (descendants of interracial marriages between Europeans and Native Americans) to wage war against Spanish rule. In Mexico and Central America, Catholic priest Miguel Hidalgo (1753-1811) is considered the starter of such movement, while the South American war of independence was won by liberators Jose de San Martin (1778-1850) and Simon Bolivar (1783-1830).

When Napoleon's Army invaded both Spain and Portugal during the Peninsular Wars (1807-1814), most criollos saw the opportunity to secede from the Spanish Empire. By 1825 most of Latin America became independent, with only some last bastions in Mexico, Peru, Chile and Argentina still being loyal to the Spanish Crown. By 1833 even these were defeated, and all attempts of reconquest by Spain had been already thwarted. Finally, in 1836 the Spanish government renounced to the sovereignty over continental America, with the exception of Cuba an Puerto Rico, which were lost during the Spanish-American War (1898).

Ill-advised immigration restrictions and economic crises in Mexico qualify as the main culprits of such immigration explosion. The specifics of why the U.S. Southwest is the main destination of Mexican immigrants is because until 1848, all the area encompassing California, Nevada, Utah, Arizona, New Mexico and Texas, as well as parts of Wyoming, Colorado, Oklahoma and Kansas used to belong to Mexico.

Before the 20th century, American borders were quite porous: many people from Mexico came and went into the United States to work temporarily -- especially in the fields as laborers or cowhands -- while returning to Mexico during the Winter months. This was more apparent in border cities, such as El Paso (TX), San Diego (CA) or Brownsville (TX).

After the Immigration Act of 1924, immigration into the United States became restricted to a certain amount of people per year; this in turn restricted the possibility of working temporarily within the United States, so many laborers found the best way to continue earning a decent wage, was to fully immigrate into the United States, bringing in their families with them, rising from 105,200 in 1900 to 624,400 in 1930.

During and after the Second World War, after the Bracero Program implementation, this immigration of Mexican workers increased, and after the passage of the Immigration Reform and Control Act of 1986, crossing the border became much more difficult, resulting in immigrants settling permanently within the United States: between 1990 and 2010 more than 7.5 million Mexican immigrants - many of them illegal - entered the U.S.
Please rewrite. Your question doesn't give us the century you ask about.