Trump’s wall would likely cost billions, but the president has said the U.S. government would pay for the structure only until Mexico, inexplicably, reimburses the costs.

If the wall is built, it would only be the most recent instance of further militarizing the U.S.-Mexico border, which already has about 650 miles of fencing.

Congress established the Border Patrol in 1924, the same year the Immigration Act of 1924 called for a limit in the number of immigrants who entered the United States via a national origins quota. Plenty has changed since, but the Border Patrol’s efforts to stop illegal immigration has grown dramatically over the years.

In 2003 the U.S Border Patrol became a part of the U.S. Customs and Border Protection, a component of the Department of Homeland Security, which was formed after 9/11. A few years later, then-President George W. Bush signed the Secure Fence Act of 2006, adding hundreds of miles onto the fence at the southwest border.

Here are 27 photos that offer a glimpse into the U.S.-Mexico border’s history.

May 1920s Philipp Kester/ullstein bild via Getty Images U.S. border guards check entering Mexicans

May 1920s ullstein bild via Getty Images U.S. border guard and Mexicans behind the border fence.

March 21, 1929 Hulton Archive via Getty Images A line of cars carrying Mexicans over the border into California. The fence in the foreground is the border; the line of cars is in the main street of Mexicali.

1930s Keystone-France via Getty Images A flock of sheep at the border between Mexico and the United States.

June 1937 Dorothea Lange/Library of Congress A pic of state border plant inspection maintained by the United States Department of Agriculture between Mexico and the United States. Shoppers returning from Mexico (Juarez) to the United States (El Paso) over the bridge that carries all the traffic are required to open their packages for inspection.

June 1937 Dorothea Lange/Library of Congress Crossing the international bridge between Juarez, Mexico and El Paso, Texas.

June 1937 Dorothea Lange/Library of Congress Mexicans entering the United States via the United States immigration station at El Paso, Texas.

1943 The LIFE Picture Collection/Getty Images U.S. soldiers exchanging money at the U.S.-Mexico border.

March 15, 1950 Michael Ochs Archives via Getty Images A view of the Sigma Pi sorority crossing under the Mexico border sign to Tijuana, Mexico in Calexico, California.

1950s Keystone-France via Getty Images Pregnant woman at the border.

1954 J. R. Eyerman/The LIFE Picture Collection/Getty Images Mexican farm laborers standing on the Mexican side of the border trying to get into the U.S.

1954 Bettmann via Getty Images Mexican workers waiting just inside the U.S. border to be let in.

October 1, 1962 James P. Blair/National Geographic/Getty Images A border guard checks passes of Mexicans entering the United States near Nogales, Mexico.

1967 Shel Hershorn via Getty Images Mexican workers crossing the border into Texas have their papers checked.

Sept. 22 1984 Bettmann via Getty Images Suspected undocumented immigrants cross into the U.S. from Tijuana, Mexico.

1990 Richard Perry via Getty Images Men look across to the other side of the Tijuana border.

1990 Serge Attal via Getty Images Woman walking along the America-Mexico border, near Tijuana.

1993 David Turnley via Getty Images U.S. Customs agents patrolling the border.

1994 Gregory Smith via Getty Images At first light, undocumented immigrants wait to cross over into the United States.

1994 Gregory Smith via Getty Images Undocumented immigrants wait on the other side of Mexico.

Aug. 19, 1997 HECTOR MATA via Getty Images Esther Pereyra Rubalcaba (left) kisses her daughter Patricia through the wall separating the US and Min Tijuana, Mexico.

July 20, 2005 David McNew via Getty Images Migrant activists lean against the border fence to pay homage to undocumented immigrants who died crossing over.

Aug. 28, 2005 Sandy Huffaker via Getty Images Young Mexican nationals peer through the border wall at the beach along San Ysidro, California.

June 6, 2015 Jose Luis Gonzalez / Reuters Paper doves in the shape of a heart are seen at the border fence in Ciudad Juarez, Mexico.

Sept. 25, 2016 John Moore via Getty Images Maria Rodriguez Torres, 70, looks towards her departing grandchildren after seeing them for the first time at the U.S.-Mexico border fence in Tijuana, Mexico. She had traveled with family members from Mexico City to see her grandchildren through the fence at "Friendship Park."

Oct. 6, 2016 Edgard Garrido / Reuters A burnt car is seen next to a section of the wall separating Mexico and the United States in Tijuana, Mexico.

Feb. 4, 2017 Justin Sullivan via Getty Images A visitor stands next to the U.S.-Mexico border fence at Friends of Friendship Park in San Ysidro, California.