(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.)

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