A country insisting on self-sufficiency will have to choose between shortages or war. In a world of protectionism, countries will not have access to resources unless those resources lie within its national boundaries. It is this basic problem with protectionism that makes extending national boundaries, and ultimately war, so appealing. 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.