In August 2010 President Barack Obama brought Gen. James Mattis, then head of U.S. Central Command, to the White House. When Obama asked him what his priorities would be, Mattis famously answered, "I have three: Iran, Iran, and Iran." But when asked in May this year what worried him most as secretary of Defense, his answer was very different: "The lack of political unity in America."

At root, what Mattis is talking about is the question of whether or not Americans still share a sense of national identity and pride. Do we all still believe that the United States is something bigger and more important than political parties or who's president, a bold experiment in individual liberty and freedom worth fighting and dying for?

As Mattis suspects, increasingly the answer is "not really."

Historically, American unity — at least regarding international affairs — expressed itself through broad public commitment to three related beliefs. Most fundamentally, Americans have long agreed that the United States is an exceptional nation with a unique role to play in global affairs. Second, Americans felt that the United States, as an indispensable nation, should be an energetic leader in the international arena. Finally, the public had confidence in the political process and trusted the institutions of government to faithfully conduct affairs abroad in the national interest.

Today, however, polls reveal that although most Americans remain patriotic, successive generations of Americans have grown up less confident in American exceptionalism. According to a 2012 Pew study, while 64 percent of the Silent Generation (born between 1928 and 1944) believes the United States is "the greatest country in the world," just 32 percent of the Millennial Generation (born between 1980 and 1998) thinks so.

Recent polls have also shown fading support for an internationalist foreign policy. A 2013 Pew poll found, for example, that for the first time a majority (52 percent) of Americans feel that the United States "should mind its own business internationally…" And though the exact number fluctuates with the crises and conditions of the moment, here too the long-term trends point down. A 2012 Chicago Council on Global Affairs study revealed that 52 percent of Americans under 30 years old thought it would be best for the United States to "stay out of world affairs" compared to just 31 percent of Americans 60 or older.

Finally, Americans' trust in the federal government has plummeted since the 1960s. The percentage of Americans who believe that the federal government can be trusted to "do what is right" has dropped to 20 percent in 2012. And when it comes to foreign policy, a 2016 Gallup survey found that just 11 percent have a great deal of "trust and confidence" in the government's handling of international problems.

The sources of these changes are varied and complex. Fatigue from 15 years of intervention in the Middle East and divisiveness in Washington politics is part of the story, but at least three historical factors also loom large. First, as memories of World War II fade, so too does the moral clarity and national pride that winning the war gave the country. Seventy years later, far more Americans carry the Vietnam and Iraq wars around in their heads as a template for war than World War II. Confidence in America's moral standing and the utility of war have taken a hit as a result, especially among younger Americans.

Similarly, the confidence that the United States drew from its dominant economic position and its successful rebuilding of Europe after the war has slowly diminished, replaced now with growing fears of economic stagnation and the perils of globalization. While still accounting for more than a fifth of the world's GDP, the United States no longer dwarfs all members of the United Nations combined. Unsurprisingly, then, a 2016 Pew study found, for example, that 49 percent of the public thought that U.S. involvement in the global economy was a bad thing, compared to 44 percent who thought it was a good thing.

Finally, the collapse of the Soviet Union spelled the end of the existential threat that had galvanized national unity for decades. Without the communist threat, the American public no longer had the strategic need or compelling justification for robust internationalism as it did during the Cold War.

Mattis may have it right. The real threat might not be Russia, Iran, or even North Korea. The most difficult task facing American leaders may just be getting the public to meet the next great challenge.

Trevor Thrall is a senior fellow in defense and foreign policy at the Cato Institute and associate professor at the Schar School of Policy and Government at George Mason