The devastated city of Hiroshima days after the first atomic bomb was dropped on Aug. 6, 1945. (AFP/Getty Images)

On Friday, President Obama becomes the first sitting U.S. president to visit the Japanese city of Hiroshima, upon which the United States dropped an atomic bomb on Aug. 6, 1945. Obama — who has spoken out about nuclear non-proliferation many times in the past — will likely use the trip to highlight the evils of nuclear weapons, though he is not expected to apologize for the bombing of Hiroshima itself.

Obama's views on the bombing of Hiroshima (as well as the later bombing of Nagasaki) may well echo that of the broader American public. In data compiled by Pew Research Center that was released last year, there was a clear shift from approval and support for the bombings to more skepticism and doubt about their use.

For example, in one poll taken shortly after the bombs were dropped, 85 percent of Americans approved of the act. By 2005, that figure had dropped to 57 percent. A poll from 2015 had found that just 56 percent thought that the bombing was justified. Among the younger generation (18 to 29 years old) only 47 percent thought it was justified.

There are a number of complicated reasons for this shift, but one is simple: More time has elapsed since World War II, providing greater distance from the horrors of a truly global, all-out war. As many have pointed out ahead of Obama's trip, some historians argue that atomic bombs were ultimately the quickest way to get Japan to surrender and that it helped avoid a grueling invasion that may well have been even more bloody. (A thornier question, however, is whether the bombs should have been dropped on civilian targets with no specific warning.)

It's worth remembering that one poll conducted by Roper immediately after World War II's end found 23 percent of Americans wished that the United States had dropped more bombs on Japan before they surrendered.

This gets to a troubling question: In what theoretical circumstances would the United States, the only country to ever use nuclear weapons on another country, ever use nuclear weapons again? One 2002 Chicago Council on Foreign Relations poll found that 22 percent of Americans said that the United States should not use nuclear weapons in any circumstances, 55 percent said they should only be used in response to a nuclear attack, and 21 percent said that in certain other circumstances the use of nuclear weapons was justified. A different Gallup/CNN/USA poll from 2005 found that 27 percent of Americans were willing to use nuclear weapons against terrorists, while 72 percent were not.

One of the most ambitious attempts to understand American attitudes to a potential nuclear war also produced some of the most shocking results. In 2013, Daryl Press and Benjamin Valentino of Dartmouth College and Scott Sagan of Stanford released a paper that sought to evaluate how strong the "nuclear taboo" against using atomic bombs really was. Their research, which polled a large number of Americans and presented a theoretical situation, found surprising approval for the United States using nuclear weapons.

When respondents were shown a potential scenario in which the United States was considering a strike against al-Qaeda militants in Syria who were attempting to use a nuclear bomb, a little under 19 percent suggested that they would prefer a nuclear strike against the militants rather than a conventional strike — even though they were told conventional strikes would be just as effective. Moreover, almost 48 percent said they would approve of a nuclear strike if it happened, versus 52 percent who would disapprove. The results became more pronounced when respondents were shown a theoretical situation in which a nuclear strike on al-Qaeda in Syria had already happened: 68 percent said they approved; 55.5 percent said the strike was ethical.

Sagan and Valentino followed up on this poll with new research in 2015. They wrote about the results in the Wall Street Journal last week. In a new poll, they presented an imaginary scenario of an Iranian attack on a U.S. carrier in the Persian Gulf that killed 2,403 military personnel. U.S. generals then gave the president two choices: invade Tehran, potentially killing around 20,000 American soldiers in the process, or drop a nuclear bomb on Tehran and kill around 100,000 Iranian civilians. The situation was designed to parallel the choices before the Hiroshima bombing, right down to the death tolls (and readers were reminded that Iran, like Imperial Japan, did not have its own bomb).

The poll found that 59 percent of respondents backed the use of the nuclear weapon, even when the potential death toll was increased to 2 million Iranians in a subsequent revision. In fact, even when presented with a diplomatic option to end the conflict, 40 percent still favored dropping the bomb. As Sagan and Valentino note, this doesn't mean in that scenario that the United States would actually drop the bomb: That decision would come from the president, not a public referendum. However, it does show that there is no real taboo among Americans about using the bomb. "Today, as in 1945, the U.S. public is unlikely to hold back a president who might consider using nuclear weapons in the crucible of war," the authors conclude.

That's a scary thought. And another detail makes it a little scarier: Polls show many Americans also believe an apocalyptic nuclear war is still possible. In a 2014 poll for 60 Minutes/Vanity Fair, 35 percent of Americans were found to most fear that a nuclear war could end humanity. That was the most of all options presented. (Only 15 percent feared that global warming could.)

More on WorldViews

It’s not just Hiroshima: The many other things America hasn’t apologized for

In Japan and America, more and more people think Hiroshima bombing was wrong

What it was like to survive the atomic bombing of Hiroshima