Concertina wire lines the top of a wall at the San Ysidro port of entry Friday, Nov. 16, 2018, in San Diego. As thousands of migrants in a caravan of Central American asylum-seekers converge on the doorstep of the United States, what they won’t find are armed American soldiers standing guard. Instead they will see cranes installing towering panels of metal bars and troops wrapping concertina wire around barriers while military helicopters fly overhead. (AP Photo/Gregory Bull)