According to Pew Research, the share of voters who said that the United States provides “too much” support to Ukraine more than quadrupled between March 2022 and December 2023, going to 31 percent from 7 percent. Among Republicans, the share grew to 48 percent from 9 percent.

A Gallup poll found even more opposition to American aid for Ukraine among Republican and independent voters. The share of Republicans agreeing that the “United States is doing too much to help Ukraine” rose between August 2022 and October 2023 to 62 percent from 43 percent and among independents to 44 percent from 28 percent.

Both polls reflect the rapid increase of isolationism in the American electorate.

I asked a range of foreign policy experts to explain what was driving the shift.

Gordon Adams, a professor of international relations at American University in Washington, D.C., and a fellow at the Quincy Institute for Responsible Statecraft, attributed the growing reluctance to support foreign engagement to the history of the past several decades. In an email, Adams made the case that

the key international events of the past 35 years have been the fall of the Berlin Wall/the disintegration of the Soviet Union, the strategic disaster of U.S. interventions in Iraq and Afghanistan and the inevitable rise of Chinese power. Together these events have changed the universe in which U.S. power is used, the reputation/influence of the United States in the global system and the willingness of the American people to support long-term engagement of the U.S. military overseas.

A combination of “bad decisions and inept execution,” in Adams’s view, has

continued to diminish the U.S. role — failure in Iraq, failure in Afghanistan, the demise of democratic movements in the Middle East (heavily supported by the United States), Libya’s collapse into anarchy, the rise of destabilizing extremism in Africa (despite rising U.S. funding and presence to counter terrorism). U.S. influence in the Middle East is clearly declining, as the actions of the Saudis, U.A.E., Qatar, Turkey and today Israel amply demonstrate. The Iranian regime is not isolated internationally but plays a clever game both in the region and with convenient allies, like Putin’s Russia. The U.S. secretary of state travels not as a power broker but a pleader seeking better behavior from once-solid allies who are no longer responding.

Because of these developments, Adams continued,

a public tired of external interventions has turned inward. There is little expectation that the United States can turn global situations around. There is no longer a political price to be paid for failing to support long-term commitments or interventions.

The world’s most capable military, Adams wrote, “is the only remaining tool the president has, and he can no longer deploy and use it effectively — it is unpopular at home to do so and unwelcome abroad (especially in the Middle East) as a force of stability.”

Some experts in foreign policy argued that declining support for American involvement in world affairs has come at a particularly bad time.

Charles Kupchan, a professor of international affairs at Georgetown, emailed me his answer to my queries:

The relative stability of the post-Cold War era is eroding as great-power competition heats up. The international system is entering a period of transition; power is shifting from West to East and from North to South.

“Because such transitions in the distribution of power are usually accompanied by instability and war,” Kupchan argued,

the United States needs to continue playing a dominant role in shaping and anchoring world order. But times have changed. The United States and its democratic allies no longer enjoy pronounced ideological and material dominance.

Amid growing tensions with China, Kupchan wrote in an article in The Atlantic in June, the United States “must factor in its own political weakness.” In the post-World War II Cold War, “the West was, for the most part, politically healthy. Ideological moderation and centrism prevailed in liberal democracies on both sides of the Atlantic, buttressed by broadly shared prosperity.”