1851: Marble monument (No. 258) erected to mark U.S.-Mexico boundary (above) Story Unity, divisions mark San Diego-Tijuana border’s history
1867: Alonzo Horton begins development of new San Diego in present-day downtown
1871: First U.S. customs inspectors assigned to border
1874: Mexican customs house established in Tijuana
1884: U.S. and Mexico treaty signed to maintain boundary line
1885: Transcontinental railroad arrives in San Diego; Tijuana Hot Spa and Hotel opened by P.L. Carle in response to Southern California real estate boom
1887: Baja California peninsula divided into two Mexican federal territories
1889: Settlement founded on Rancho Tía Juana

1904: Mounted guards patrol southwest border
1909: San Ysidro founded
1910: Mexican Revolution begins; U.S. erects barbed wire cattle fence at San Ysidro; San Diego and Arizona Railway reaches Tijuana
1911: “Magonista Revolution” occupies Tijuana, threatening to bring about Baja’s annexation to the U.S.
1915-16: Start of the Panama-California Exposition in Balboa Park; Tijuana Fair and horse racing track opens
1917: U.S. enters World War I
1920: Prohibition creates a boom for Tijuana
1921: Emergency Quota Act excludes Mexican workers
1924: U.S. Border Patrol established
1926: Border curfew enacted at San Ysidro, lasts until 1933
1928: Agua Caliente Resort and Casino opens; closes in 1935
1929: Tijuana officially named
1930: Great Depression triggers Mexican repatriation

1935: Start of California-Pacific International Exposition in Balboa Park
1937: Free-trade zone declared in Baja
1938: Joint International Outfall sewer pipe built
1942: Bracero Guest Worker program established, 219,000 Mexicans employed as agricultural workers through 1947
1947: Colorado River water imported to San Diego; Jai-Alai palace opened in Tijuana

1952: Baja California (North) granted Mexican statehood
1954: U.S. rounds up and deports more than one million immigrants to Mexico
1955: Chain-link fence separates San Ysidro from Tijuana
1957: San Ysidro annexed by San Diego
1964: Bracero program ends
1965: San Diego agrees to treat Tijuana sewage on emergency basis; maquiladora program initiated in Tijuana
1969: Operation Intercept mounted to deter crime and it disrupts border traffic; San Diego celebrates 200th anniversary

1972: Tijuana River channelization project begins; squatters forced to relocate
1978: “Torilla Curtain” border fence erected
1981: San Diego Trolley south line opens service from downtown San Diego to San Ysidro
1982: Mexican peso devaluation
1984: Centro Cultural opens in Tijuana
1985: Otay Mesa border crossing opens
1986: Immigration reforms grant amnesty to resident immigrants, penalties are imposed on employers of undocumented workers
1989: Tijuana celebrates 100th anniversary; “Light up the Border” citizens group protests illegal immigration
1990: U.S., California and Mexico sign agreement for construction of new Tijuana sewage treatment plant; Border Patrol erects 14-mile solid steel barrier near San Diego

1992: First InSITE binational visual art project
1993: Caltrans erects a fence on I-5 median at San Ysidro to discourage migrants from running across the freeway
1994: Presidential candidate Luis Donaldo Colosio assassinated in Tijuana; NAFTA takes effect; Operation Gatekeeper launched
1995: SENTRI trusted-traveler verification program begins
1996: Illegal Immigration Reform and Immigrant Responsibility Act authorizes construction of a 14-mile, triple-layered fence near San Diego
2006: Secure Fence Act calls for 700 miles of border fencing
2008: Anti-crime Mérida Initiative signed
2009: Work begins on San Ysidro border crossing expansion
2012: Mexico launches NEEC cargo certification; El Chaparral border crossing opens
2014: San Ysidro Port of Entry opens 25 northbound lanes and 46 inspection booths