A U.S. border patrol agent on the U.S. and Mexico border near Sunland Park, New Mexico.

President Donald Trump on Friday declared a national emergency to build a border wall. Yet illegal crossings at the U.S.-Mexico border are at their lowest levels in nearly 50 years.

That reality calls into question Trump's justification for declaring a national emergency over the border, potentially setting up years of legal challenges.

According to data from U.S. Customs and Border Protection, there were 303,916 apprehensions along the southern border during the 2017 fiscal year, the lowest level since 1971. Illegal immigration at the border peaked in 2000, when there were more than 1.6 million apprehensions.

Trump has frequently referred to the southern border as a "crisis," despite the fact that the majority of illegal immigration occurs when immigrants overstay their temporary visas.

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Congress passed a spending bill Thursday night to avert another government shutdown. The bill allocates $1.375 billion for 55 miles of border fencing, far less than the $5.7 billion Trump had requested.

White House press secretary Sarah Huckabee Sanders said in a statement Thursday that while Trump will sign the bill, he "will also take other executive action – including a national emergency – to ensure we stop the national security and humanitarian crisis at the border."

Declaring a national emergency is seen as a last-ditch effort for Trump to fulfill one of his main campaign promises. Opponents will likely challenge the measure in court and Democrats could take up a resolution to try to block it in Congress.