Iraq war veteran Evan Cohen holds up a sign during a protest at San Francisco International Airport in San Francisco, California on January 29, 2017. AFP PHOTO / Josh EdelsonJOSH EDELSON/AFP/Getty Images

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.

Conquest is no longer seen as a good thing

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.

Even when there has been a blatant attempt to take something by force, other states have usually looked to reaffirm the principle that plunder and grabbing other states’ territory are wrong. Condemning the spoils of war makes it possible to form coalitions around violations of the norm against conquest and thus helps to mobilize resistance. Famously, the U.N. rejected Iraq’s conquest of Kuwait in 1990 and authorized a U.S.-led coalition to overturn the occupation by force. The most recent example is the widespread international condemnation of Russia’s annexation of Crimea in 2014. When Russia vetoed a widely-supported U.N. Security Council resolution which reaffirmed the territorial integrity of Ukraine, it was greeted with economic sanctions.

Even when states try to bend the rules – as when Russia annexed Crimea, or invaded Georgia in 2008 and established the de facto state of South Ossetia, they usually justify what they are doing as providing local populations with self-determination. Russian President Vladimir Putin appealed to the democratic will of the people of Crimea as expressed in a referendum (despite numerous irregularities) when he approved the annexation.

That may be changing, thanks to Trump

Trump very clearly has a different point of view. Will his attitude change the perspective of other states? One example from history hints at the potential damage that could be done by powerful actors who reject the norm against conquest. In 1935, Italy’s Benito Mussolini invaded Ethiopia, also called Abyssinia. The international community condemned the invasion, swore to uphold the peace and to defend the territorial integrity of Ethiopia, a member of the League of Nations, and coordinated economic sanctions on Italy. However, a group of policy makers in the U.K. and France entered into negotiations with Rome to try to end the violence through concessions to Mussolini. Instead of preserving Ethiopia against aggression, the U.K. and France offered vast amounts of territory and a trade monopoly in exchange for a halt to the war.

When this agreement was leaked and became public, the world did not see it as a necessary adjustment to political realities. Instead, people thought that Mussolini, an aggrandizing warmonger, was being rewarded for victories on the field of battle. Many states saw this as betraying the norm that conquest and plunder were a thing of the past. Even though the U.S., the Soviet Union and a few other states refused to recognize the Italian Empire in East Africa, it was enough that the consensus was broken. Some states adopted strict neutrality, refusing to treat aggressors any differently from victims. Some states went further and tried out appeasement as a way to deal with aggressive states, such as Hitler’s Germany. That didn’t work out very well.

So, merely raising the prospect of the spoils of war likely has a corrosive effect on people’s expectations. In addition to plundering Iraq’s oil, Trump has specifically mentioned that he wants to lift sanctions on Russia and would consider recognizing the annexation of Crimea. If Trump condones plunder and conquest under the maxim, “to the victor go the spoils,” we might see a similar disintegration of the will to protect the current peaceful global order. Opportunistic states will see their chance to grab some long-coveted territory, calculating that if the U.S. is not committed to resisting conquest they will face little coordinated opposition. Small states, fearful of facing acquisitive invasion, will feel forced to arm themselves and form alternative alliances that do not rely upon the now doubtful support of the U.S. The result will be increased militarization of international politics, instability, uncertainty, and a return to a world with the ever-present threat of war.