The border is nearly 2,000 miles long and covers four U.S. and six Mexican states. It's the most frequently crossed international border in the world, with some 350 million people crossing legally every year, the U.S. Embassy in Mexico says.
1836
Texas gains independence from Mexico; Rio Grande becomes its southern border.
1848
At the end of the Mexican-American War, officials sign the Treaty of Guadalupe-Hidalgo, in which Mexico gives up nearly half its territory, including modern-day California, most of Arizona and New Mexico and parts of Colorado, Nevada and Utah.
1853
Under the Gadsden Purchase, the United States pays $10 million for about 30,000 square miles that runs south of the Gila River, east to El Paso and west to California. The purchase includes Tucson.
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1904
The first border patrol is established to stop Asian workers from entering the U.S. through Mexico.
1910
The Mexican Revolution begins. Thousands of Mexicans cross the border for safety.
1921
The Immigration Act of 1921 restricts the immigration of Southern and Eastern Europeans. Agriculture lobbyists rally to exempt Mexicans.
1924
The Immigration Act of 1924 establishes the U.S. Border Patrol, halts the flow of other immigrant groups, establishes border stations to formally admit Mexican workers and sets a tax on each entry.
1942
The Bracero program begins in response to worker shortages brought on by World War II, allowing Mexicans to work temporarily in the United States, mostly in agricultural areas.
1953
"Operation Wetback" takes place, during which the U.S. Immigration Service deports more than 3.8 million people of Mexican heritage.
1963
The United States and Mexico sign the Chamizal Treaty to divide land around the Rio Grande in El Paso. As part of the treaty, a 167-foot-wide, 15-foot-deep concrete channel is constructed to prevent the Rio Grande from blurring the international boundary in the future.
1964
The Bracero program is repealed.
1986
Congress passes the Immigration Reform and Control Act, granting amnesty to an estimated 3 million illegal immigrants and making it illegal to knowingly hire illegal workers.
1993
Then-El Paso Border Patrol chief and current U.S. Rep. Silvestre Reyes, D-Texas, puts into place the first deterrence strategy along the border with "Operation Hold the Line."
1994
NAFTA — the North American Free Trade Agreement — goes into effect, stimulating U.S.-Mexican trade. Massive increases in border populations occur.
Operation Gatekeeper begins in San Ysidro/ Tijuana, the beginning of Border Patrol's national deterrence strategy.
2001
Attacks on the World Trade Center and the Pentagon cause the country to re-examine immigration policies.
2003
The Department of Homeland Security becomes operational, incorporating parts of 22 agencies, including the Immigration and Naturalization Service, which is abolished; immigration functions are divided between Customs and Border Protection, Immigration and Customs Enforcement, and Citizenship and Immigration Services.
2005
House Bill 4437 is passed, calling for 700 miles of wall along the U.S.-Mexico border at an estimated cost of $2.2 billion, and making illegal crossing into the United States a felony.
The governors of Arizona and New Mexico declare states of emergency in their counties along the border.
2006
Senate Bill 2611 is passed, calling for 370 miles of fencing and 500 miles of vehicle barriers on the border and 2,400 more Border Patrol agents per year through 2011. It also creates a guest-worker program.
Sources: PBS — The Border; Migration Policy Institute; Encyclopedia Britannica
