The border wall was a central 2016 campaign promise by Trump and is part of his 2020 re-election bid.
Trump, a Republican seeking re-election on Nov. 3, has said the wall is needed to curb illegal immigration and drug trafficking across the southern border. Democrats have called the wall immoral, ineffective and expensive. Trump promised during the 2016 presidential race that Mexico would pay for the wall. Mexico refused.
In 2019, Trump's administration redirected $2.5 billion from military counter-narcotics programs for border wall construction in California, New Mexico and Arizona.
The Southern Border Communities Coalition, a group advocating for people living in border areas, joined the Sierra Club environmental group in suing to try to block Trump's action. The challengers have said the wall would be disruptive to the environment in part because it could worsen flooding problems and have a negative impact on wildlife.