Who is Kevin McAleenan, Trump's acting Homeland Security chief after Kirstjen Nielsen leaves?

President Donald Trump's pick to head the Department of Homeland Security didn't take the traditional route to his position, but Kevin McAleenan has carried out every aspect of the president's hard-line immigration strategy in an attempt to seal the southern border.

With Sunday's resignation of Kirstjen Nielsen, the former business and corporate lawyer steps into a perilous position where he'll have to please an increasingly exasperated president just as illegal migration along that southern border continues to explode.

Nielsen's departure, and McAleenan's ascension, come as a surge of migrants has...