The data behind Trump's new border strategy

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(CNN)Last summer, President Donald Trump repeatedly boasted about the downturn of illegal crossings at the US border with Mexico.

Though he'd only been in office for a few months, Trump -- much in the way he discusses the stock market -- took credit for the new numbers and claimed, without evidence, that his administration was succeeding where its predecessors failed.
Fast forward about a year and Trump is reversing course, sounding the alarms after a March spike in border arrests, and calling for a military deployment to prevent what he's described as an imminent flood of illegal crossings.
So, what changed?
    To start, Trump was triggered by new illegal border crossing numbers and reports, played up on Fox News, of a migrant caravan headed north toward the US from Central America. The March 2018 southwest border apprehension stats, though three times higher than in 2017, track with historic patterns and recent years. (The typically mild weather at this time of year, making the trip less dangerous, is as good an explanation as any.)
    Last year's decline could be attributed to any number of variables, including the specter of a crackdown by the new Trump administration. The number of apprehensions also greatly increased ahead of the initial dip. These nuances and unanswered questions didn't feature in Trump's weekend Twitter rant, nor his call on Tuesday for a renewed military presence on what he derided as the country's "'Weak Laws' Border."
    The long-term statistics suggest an even less dramatic story. Arrests of people trying to slip across the border illegally have been trending down since 2000. Trump himself acknowledged as much, without crediting his past administrations, on Thursday morning, when he tweeted about apprehensions reaching a 46-year low (by the end of fiscal year 2017).
    Trump's remarks on Tuesday likely provided some better insight into his current mindset.
    "Until we can have a wall and proper security," he said during a meeting with Baltic leaders, "we're going to be guarding our border with the military."
    His frustration with the congressional spending bill, which didn't provide new funding for his pet project, is no secret.
    In a presidency colored by reaction, it's best to understand the current debate in these purely political terms. Trump opted against a bargain that would have funded wall construction. Now, with nothing to show a base that views the wall as his signature campaign pledge, he is attempting to flex a strained muscle.
    But even then, ordering the National Guard to the border is hardly a novel plan. President George W. Bush did it in 2006 as part of $1.3 billion effort to bulk up enforcement mechanisms. In 2010, President Barack Obama picked up the ball and kickstarted a program that would exist in some form through his second term.
    Trump said on Thursday he was considering plans to send "anywhere from 2,000 to 4,000" National Guard troops to the border -- and keeping them there until his proposed wall is complete. That number would come in under Bush's total and slightly above Obama's initial number.
    The details after that are a bit less certain, as the administration works to keep pace with the President's rhetoric.