Video report by ITV News correspondent Emma Murphy
Donald Trump has declared a national emergency, using executive powers to bypass Congress and obtain more money for his much-promised border wall.
In a speech at the White House on Friday, the US president said the declaration would free up $8bn for the project.
"We're going to confront the national security crisis on our southern border," Mr Trump said.
"We have an invasion of drugs, invasion of gangs, invasion of people, and it's unacceptable.
"Everyone knows that walls work."
Despite Mr Trump's claims, that the number of illegal immigrants from Mexico in the US has dropped by 1,000,000 since 2006.
Mr Trump also claims that 4,000 terrorists crossed the Mexican border in 2018 but statistics for the first half of the year has the figure at just six.
His press secretary, Sarah Huckabee Sanders, tweeted a photograph of Mr Trump signing the 'national emergency' declaration moments before his news conference.
He told stories - without providing evidence - of women suffering abuse as they were smuggled across the border from Mexico, and argued that the border wall would help tackle drug cartels.
He said he fully expects legal challenges, but insisted that he believes his decision will be justified.
"Sadly, we'll be sued and sadly it will go through a process and happily we'll win," he said.
House Speaker Nancy Pelosi and Senate Minority Leader Chuck Schumer have already slammed the move as "unlawful", stating in the hours following the speech that they would take action "in the Congress, in the Courts, and in the public."
They argued it would "shred the Constitution" by usurping Congress' power to control spending.
White House officials have said that some of the money would come from military construction projects.
But Pelosi and Schumer said Trump would be using money needed "for the security of our military and our nation."
He used the same speech to promise a "substantial increase" in trade between the UK and the United States.
Mr Trump said despite the "complexity and problems" surrounding Brexit, the two countries had agreed to "preserve" the trade arrangement - with more to come in future.
Mr Trump also used the speech to state there would be a further announcement on Syria in the next 24 hours, with many expecting the withdrawal of US troops from the country to be confirmed.
"We have a lot of great announcements having to do with Syria and our success with the eradication of the caliphate," Mr Trump said.
Potentially withdrawing US troops from Syria has been met with resistance from the country's top general, Joseph Votel, who thinks it would be a mistake.
Speaking to CNN, General Votel said: "[IS] still has leaders, still has fighters, it still has facilitators, it still has resources, so our continued military pressure is necessary to continue to go after that network."
Mr Trump took time to praise China's policy to use the death penalty for drug traffickers, with the president suggesting the US could solve their problem by using the same means.
The president explained how in China drug dealers get "a thing called the death penalty. Our criminal drug dealer gets a thing called 'how about a fine?'"
"So if we want to get smart, we can get smart. You can end the drug problem. You can end it a lot faster than you think."