The use of force has undergone profound changes since the 19th century. Back then, military victory meant that you got to keep the spoils of that war. That might be gold, jade, silk, and porcelain, which British and French soldiers took from Beijing in the Second Opium War of 1860, or it might be Alsace-Lorraine, taken by Germany from France in 1871. Great powers were less inhibited in going to war because they knew that they would likely face no coordinated opposition to their ill-gotten gains. Might doesn’t make right under modern international law, but it used to.

Today there is an international consensus that conquest is not okay. However, this consensus might be in jeopardy. As president of the United States, Donald Trump will have an outsized influence on what people around the world see as the rules of the international order. And Trump seems to reject the prohibition on the spoils of war. Both before the presidential election and afterwards, Trump has justified his desire to take Iraq’s oil using the phrase, “to the victor belong the spoils.” This may herald the return to predatory behavior and a more violent world.

Starting with the League of Nations in 1920, world opinion has, in fits and starts, moved away from the idea that conquerors have rights and toward the view that any disputes are best solved through diplomacy and peaceful change.