Resource shortages caused by protection have been a problem in Europe for centuries. Mises observes that it was a problem in particular for Germany. Protectionism makes war profitable. Where borders are open it makes no material difference to the citizens how far national boundaries stretch. Goods and services will still flow freely.
Citizens can migrate and sell their labor anywhere when borders are open.
If so, trade is a solution to war. First, in contrast with protectionism, trade makes war more costly to the common people. Citizens of democracies will be less likely to vote for leaders who wage war. Second, free trade tends to undermine autarky. It spreads liberty and helps create new democracies, thus empowering those citizens to whom war is most costly.