The Pentagon will increase the number of active-duty troops along the U.S.-Mexico border to about 6,000 by the start of next month, a return to the peak force size deployed shortly after the mission began last fall, a senior U.S. defense official said Friday. About 1,000 additional personnel will join the roughly 5,000 already operating on the border, amid orders to string more concertina wire and install detection systems in remote areas away from official ports of entry, the senior defense official said. The official, who was authorized by the Pentagon to speak to reporters only on the condition of anonymity, said that so far active-duty military forces had installed 70 miles of concertina wire, reinforced ports of entry, provided medical support to migrants and helped transport Customs and Border Protection agents by aircraft. “That mission has evolved,” the senior defense official said.