At the Pentagon, several officials privately expressed concern about being seen as picking a fight with an ally at a time when the military has plenty of adversaries — the Islamic State, North Korea, Russia, Syria — to contend with. Massing American troops at another country’s border, several current and former Defense Department officials said, would send a message of hostility and raise the chances of provoking an all-out conflict.

“We are so lucky here in this country when you look at our borders,” said Maj. Gen. Paul Eaton, a retired veteran of the Iraq war. “We’ve got the Pacific on one side, the Atlantic on the other and allies to the north and the south. Mexico is not an adversary. Why would you present this offensive barrier to a friendly country?”…

[M]ilitary officials worry that Mr. Trump may not be satisfied with the Bush- and Obama-level deployments. Even limited deployments, Pentagon officials said, have come with their share of trouble.