In the past, the United States stood by its allies, championed freedom and opposed human rights abuses. No longer. But perhaps the most important principle that Trump is disregarding is one that has underpinned international law since the 1648 Peace of Westphalia. This is the doctrine of “territorial integrity,” which forbids states from expanding their borders by force.

President Franklin D. Roosevelt and Prime Minister Winston Churchill viewed territorial integrity as so important that they made it both the first principle (“their countries seek no aggrandizement, territorial or other”) and the second (“they desire to see no territorial changes that do not accord with the freely expressed wishes of the peoples concerned”) in the 1941 Atlantic Charter. This principle was then enshrined in Article 2 of the 1945 U.N. Charter.

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The makers of the postwar order felt so strongly about this issue because they had witnessed its violation by Germany, Japan and Italy in the 1930s. When President George H.W. Bush, himself a World War II veteran, rallied an international coalition to roll back Iraq’s invasion of Kuwait, he had the lessons of Munich in mind: “Half a century ago, the world had the chance to stop a ruthless aggressor and missed it. And I pledge to you, we will not make that mistake again.”

Unfortunately, the past three presidents have allowed aggressors to undermine this important norm.

In 2008, President George W. Bush did little when Russia invaded Georgia. Today, through puppet regimes in the breakaway republics of Abkhazia and South Ossetia, Russia continues to control 20 percent of Georgia’s territory.

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Emboldened, Russia in 2014 launched another war of territorial aggression against another neighbor: Ukraine. It annexed Crimea (population 1.9 million) and created front groups to control the Donbas region (population 6.6 million) with Russian military aid. Vladimir Putin is now handing out Russian passports to Donbas residents.

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President Barack Obama imposed sanctions against Russia and provided non-lethal aid to Ukraine. Trump provided lethal aid, too. But now Trump has undermined Ukraine by holding that aid hostage to its willingness to help his election campaign, by repeating Russia’s anti-Ukraine propaganda and by fawning all over Putin — even trying to invite the Russian dictator back to the Group of Seven after he was kicked out following the invasion of Ukraine.

China has gotten the message that, in the new world disorder, aggression pays. In 2016, an international tribunal in The Hague rejected China’s attempts to annex virtually the entire South China Sea. China disregarded this court ruling with little pushback from Obama and continues building and fortifying island bases. This is a slow-motion occupation — and we’re barely paying attention. Trump talks all the time about his trade war with China but hasn’t tweeted about the South China Sea since becoming president (and only once before that).

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So little does Trump care about the principle of “territorial integrity” that he recognized Israel’s annexation of the Golan Heights, captured from Syria in 1967. If Israel annexes much of the West Bank, as Prime Minister Benjamin Netanyahu has promised to do, Trump would probably recognize that, too.

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Now, Turkey has gotten into the act. President Recep Tayyip Erdogan this month launched an invasion of northern Syria that felt very different from his occasional attacks on Kurds in northern Iraq. This was not a quick in-and-out operation. Erdogan intended to create a 270-mile long, 20-mile deep “buffer zone” controlled by the Turkish army. Trump gave a green light to this aggression by moving U.S. troops out of the way, and now he has lifted all sanctions on Turkey even as its troops remain in Syria. It was Putin, not Trump, who limited the Turkish land grab to “only” about 1,500 square miles and won Erdogan’s approval to allow Syrian and Russian troops into the border area. The wishes of the area’s Kurdish inhabitants for self-rule were ignored.

The right of “territorial integrity” is not, of course, absolute. International law has long recognized that nations forfeit their sovereignty if they allow their territory to be used for aggression against their neighbors. In more recent decades, human rights advocates have also sought to carve out an exception known as R2P (responsibility to protect) if grave human rights abuses such as genocide are being committed within a nation’s territory.

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Israel’s takeover of the Golan Heights was part of a defensive military operation, but that doesn’t justify annexation, and neither of these exceptions apply in any of the other cases. Turkey cites security concerns for its offensive, but there is no evidence of Kurds launching terrorist attacks in Turkey from northern Syria. And the Turks aren’t stopping human rights abuses but, rather, are committing them.

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“This will not stand, this aggression against Kuwait,” George H.W. Bush said in 1990. That Russia, China and Turkey are now getting away with their aggression undermines international law and makes the world a more dangerous place. And rather than succor the victims of aggression, Trump is cheering on the aggressors.