During the run-up to his loss in the Wisconsin primary, Republican front-runner Donald Trump Donald John TrumpObama calls on Senate not to fill Ginsburg's vacancy until after election Planned Parenthood: 'The fate of our rights' depends on Ginsburg replacement Progressive group to spend M in ad campaign on Supreme Court vacancy MORE called NATO "obsolete." He demanded that its members reimburse the United States for having defended them or else defend themselves because "they're ripping off the United States." He explained that, "And if it breaks up NATO, it breaks up NATO." Trump also suggested that Japan and South Korea should defend themselves by developing their own nuclear arsenals. Trump's national security policy, then, is to reverse seven decades of American world leadership.

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In his book, "The World America Made," historian Robert Kagan describes how the world has changed since the 1941 Japanese attack on Pearl Harbor, which shattered isolationist illusions and conferred global leadership on the United States. At the time, there were only a dozen democracies; today there are over 100. During the four centuries prior to 1950, global gross domestic product rose by less than 1 percent a year; since then, it has averaged an annual 4 percent increase. In the first half of the 20th century, two world wars broke out, killing some 80 million people and, before then, war among leading powers was nearly constant; in the last 60 years, no great powers have gone to war with one another.

Kagan argues that this was not happenstance. It was due to the fact that, since World War II, the leading world power is a democracy possessing the largest free market-economy on the planet, a commitment to human rights and freedom, and military might far surpassing any conceivable combination of aggressive regimes. By the standards of history's empires and dynasties, this makes America an exceptional nation. It is the key to our security.

Trump doesn't remotely grasp the connection between our global leadership and our security. He thinks a profit-and-loss statement is the right metric for American greatness and security (not so coincidentally, the same metric he uses to proclaim his own greatness). Our allies should be treated like subsidiaries of one of his companies — if they are losing money, spin them off.

But the American bases in European countries, Japan and South Korea are not just protecting those nations; those bases also protect the United States. In 2013, Navy Admiral James G. Stavridis, then head of the U.S. European Command and supreme allied commander in Europe, described American installations in Europe as the "forward operating bases for 21st century security." He further pointed out that Europeans are our "most steadfast, reliable, battle-tested, and important global partners as we confront the strategic risks and military challenges of the 21st century."

Once we foolishly thought that two oceans would keep us safe. Yet twice in half a century, the United States had to send millions of its young men and women to fight in wars that started on the other side of the Atlantic Ocean. After World War I, we brought them back and retreated inward; after World War II, we knew better and kept them in Europe and Asia to maintain what is sometimes called an international liberal order. Following Trump's remarks about Japan and South Korea, President Obama appropriately pointed out that our military alliances with those countries are the "cornerstone" of American policy in Asia: "You don't mess with it. It's an investment that rests on the sacrifices that our men and women made back in World War II."

Global leadership has been a costly undertaking in blood and dollars. We have made mistakes for which we paid dearly, and, even when we do it right, more often than not the effort earns us enmity instead of thanks. But the post-World War II order, whether measured in trading partners prosperous enough to buy our goods or invest in our debt, strategic alliances available to meet threats, or democracies that share and promote our values, has served this country extraordinarily well.

Trump would pull back from that international commitment, which all 12 post-war presidents have vigorously supported, and instead encourage a world order based on the proliferation of the most destructive weapons known to mankind. It will create a frightening and chaotic world that definitely will not keep America safe.

Wallance, a writer and lawyer in New York City, is the author most recently of "America's Soul in the Balance: The Holocaust, FDR's State Department, and the Moral Disgrace of an American Aristocracy."