WASHINGTON — The Pentagon is deploying 5,200 troops to America’s southwest border in an extraordinary military operation a week before nationwide elections in which President Donald Trump wants voters to focus on a slow-moving “invasion” of Central American migrants. The number of troops being sent is more than double the 2,000 who are in Syria fighting the Islamic State group. Two caravans of would-be migrants, mostly on foot and seeking asylum, are still hundreds of miles from the U.S.