The question here is what’s cheaper — and what’s wiser. To Mr. Trump, keeping American forces at home is part of the “America First” philosophy: They would be more focused on homeland defense and less likely to get involved in conflicts abroad that drain money, and power, from the American military. But at the Pentagon, most officers and political appointees view that as pre-World War II thinking and argue that forward-deployed troops are crucial to national security.

The “freedom of navigation” tours that American forces take through the South China Sea are supposed to be a reminder to the Chinese that there could be a price to expanding the area they argue is sovereign Chinese territory and reclaiming reefs to build military installations. It would be hard to do those from home. The United States launches drones — and keeps a store of 50 nuclear weapons — at the American base in Incirlik, Turkey. The naval base in Bahrain runs patrols to keep Iran from choking off oil routes. And troops based in South Korea, Japan, Germany and Africa train with local forces.

When a tsunami hit Indonesia and earthquakes struck China and Pakistan, American forces in the Pacific delivered emergency supplies. And it was Republicans who complained that when the Benghazi attack happened in 2012, American forces were too far from Libya to help.

As for cost, there is some question of which approach would save more money. Japan, for example, pays such a large percentage of the cost of housing troops that the Pentagon has long argued it would be more expensive to bring them home.

On Trade

Mr. Trump doesn’t like the North American Free Trade Agreement and never has. His approach to dealing with it, he said, would be to renegotiate it, or “I would pull out of Nafta in a split second.”

Nafta is deeply unpopular, and in part, economists say, that is because jobs leave the country en masse. (Mr. Trump often refers to the Carrier air-conditioning plant that is shutting down and moving 1,400 jobs out of the country.) But when jobs are created in the United States, it is often hard to say whether that is because of a trade deal or just an improvement in business.

As a practical matter, no one knows what withdrawing from Nafta would mean — just as no one knows what “Brexit” means for Britain and the rest of Europe. Unwinding a huge trade agreement, and the rules that have governed tariffs, movement across borders and adjudication of disputes, would be enormously complicated. And doing so could be costly: United States businesses would lose easy access to the Mexican and Canadian markets, along with preferential treatment on investments there. But Mr. Trump insists he could negotiate a far better deal.