While the number of families coming over the border has risen sharply, the number of border arrests — the leading gauge of how many people are trying to cross illegally — is actually one-quarter of what it was in 2000, dropping from 1.6 million then to 400,000 in 2018. Also noteworthy: The contingent of active-duty U.S. troops at the border has been more than halved, dropping from a peak in the fall of about 5,900 to about 2,350 last week.