Washington (AFP) - President Donald Trump's shock order to send National Guard troops to the frontier with Mexico Wednesday came after data showed that illegal immigration has sharply rebounded following a plunge in his first year in office.

Data on border apprehension for March released late Wednesday indicates undocumented immigrants are pouring into the country at the highest level in four years.

Trump's warning of a crackdown when he entered the White House in January 2017 drove the number of apprehensions of illegal border crossers -- an indicator of total crossing numbers -- to four-decade lows.

For example, apprehensions in February 2017 were 23,555, down from more than 38,000 a year earlier. And they hit a monthly low of 15,766 in April 2017, less than one-third of the previous year's number.

At the time anti-immigration groups celebrated that as the "Trump Effect" and Trump held it up as his great success.

"Jobs are returning, illegal immigration is plummeting, law, order and justice are being restored. We are truly making America great again!" he tweeted that April.

A year later, the data suggests that the Trump Effect lasted barely seven months and that undocumented immigrants are entering the country at a rate similar to 2014-2016, before he launched his run for president on an anti-immigration platform.

Apprehensions on the southwest border in January and February totaled 72,517, compared to 66,018 a year earlier.

Numbers for March released Wednesday soared: 50,308 people were apprehended, the highest level for March since 2014. A year ago, only 16,588 people were nabbed sneaking into the United States from Mexico.

That surge, officials say, directly correlates to a similar rise in the number of people successfully entering the country without documents.

Despite boosting border staffing and spending and giving border patrol officers more power, said Homeland Security Secretary Kirstjen Nielsen, "we have recently seen the numbers of illegal border crossings rise from 40-year-lows last April back to previous levels."

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Nielsen said the rebound comes as human smugglers encourage migrants to take advantage of their knowledge of US laws to avoid quick deportation.

For example, the Homeland Security Department said that one out of ten people being apprehended by Customs and Border Protection officers asks for asylum based on fear for their lives in dangerous countries like Honduras -- compared to one out of 100 in 2013.

Nielsen said that there has been a major jump in recent years in the number of families and unaccompanied children seeking to sneak across the border, and that now half of border crossers are from Central America.

"The traffickers and smugglers know that these individuals cannot, by law, be easily removed back to their country of origin."

Even so, recent CBP data shows a sharp decline overall from one year ago in unaccompanied children and families apprehended at the border, suggesting Trump policies have had some sustained impact.

"We will not allow previous illegal immigration levels to become the norm," Nielsen said.