The purpose of President Trump’s airstrikes on a Syrian government airfield was, according to Trump, to secure the national security interests of the U.S. “and deter the spread and use of deadly chemical weapons.”

But there were apparently other, more humanitarian considerations for the president. He opened his televised remarks on Thursday with a statement about the lives that had been lost in the sarin gas attack and the cruel nature of their deaths: “It was a slow and brutal death for so many, even beautiful babies were cruelly murdered in this very barbaric attack. No child of God should ever suffer such horror.” Trump was clearly moved to action by the horror of the attack on civilians, but through the years, American public opinion has been mixed on the subject of humanitarian grounds for foreign intervention.

The early polling about Trump’s decision to strike suggests there’s broad support for it, but Syrian intervention on humanitarian grounds hasn’t been popular in the past. In the fall of 2013, when President Obama sought congressional approval for military action against Bashar al-Assad’s government after it used chemical weapons on civilians only 36 percent of Americans favored the U.S. taking military action to reduce Syria’s chemical weapons use. Fifty-one percent of those surveyed opposed military action on these grounds. (Obama’s effort failed.)

But the contours of the Syrian crisis changed when the Islamic State began to operate in the country. Americans’ calculus appears to have shifted as well. A poll from last year showed that Americans generally supported limited intervention in Syria against extremist groups — 72 percent were in favor of airstrikes against them, and 57 percent were in favor of special operations forces working in the country.

That difference — more support for acting against the Islamic State, an international terror threat, than against the Assad government’s home-turf war crimes — speaks to Americans’ fraught relationship with humanitarian intervention. Looking at the last 20 years or so of American debates over humanitarian intervention in conflict zones around the world, one sees ebbs and flows in support, reflecting the varying degrees of success of American missions abroad. (I looked at public opinion surrounding the conflicts in Somalia, Rwanda, Bosnia, Kosovo and Darfur.)

Americans started the last decade of the 20th century bullish on American intervention in humanitarian crises. President George H.W. Bush announced he was sending troops into Somalia’s civil war and famine in December 1992, and a Gallup/Newsweek poll that month showed that a majority of Americans (66 percent) approved of this move. But less than a year later, in October 1993, 18 Americans soldiers died in the Battle of Mogadishu while Bill Clinton was president. That seemed to have serious influence on American opinion of humanitarian intervention.

In June of 1994, in the midst of the three-month killing spree in Rwanda that would result in the deaths of 800,000 people, CBS News asked poll respondents if “in order to stop the killing in Rwanda” they favored or opposed sending in ground troops. Sixty-one percent opposed the effort. When the same poll asked if the “United States has a responsibility to do something to stop the killing in Rwanda,” 51 percent of respondents said the country did not have that moral obligation.

This 1990s turn against intervention — likely with the deaths in Mogadishu still fresh in Americans’ minds — continued with the Bosnian war. In early 1994, as NATO bombing began and Clinton declared the U.S. willing to aid in the effort, Americans still seemed uneasy with the idea of a moral mandate. A Time/CNN poll in February of that year asked if respondents thought “the United States has a moral obligation to protect the citizens of Sarajevo and Bosnia.” Fifty percent said no.

After the NATO action the course of events shifted bloodily and the war took on new dimensions in the global consciousness. In October 1995, the first news reports of the July massacre in the town of Srebrenica began to surface; over 7,000 Muslim men were killed over the course of a couple of days.Perhaps with the Bosnian massacres on their mind, Americans were more amenable to intervening when the war in Kosovo reared its head and NATO began a bombing campaign in March of 1999. A February 1999 Gallup poll asked if the U.S. had “a moral obligation to help keep the peace in Kosovo, or not,” and 52 percent of respondents said it did. On the same survey, respondents were asked whether they would “favor or oppose the U.S. and its allies committing a small number of ground troops in order to help establish peace in that region.” Sixty-six percent of respondents favored ground troops, a stunning turnaround from only four years prior, when the Rwandan and Bosnian questions were on the table.

The dawn of the new century and the George W. Bush administration of course changed American foreign policy — the attacks on Sept. 11, 2001, and subsequent wars in Iraq and Afghanistan left an American public overwhelmed by overseas conflict. Perhaps because of this, the killings in the Darfur region of Sudan resonated less urgently with the public. All this despite the U.S. Congress formally declaring the killings in the Darfur region of Sudan to be “genocide” and Bush also calling the Darfur killings “genocide” in 2005. But asked to rank a list of foreign policy priorities in a July 2006 NBC/Wall Street Journal poll, only 7 percent of respondents said “dealing with the genocide in Darfur” should rank as the U.S.’s No. 1 foreign policy priority. Stabilizing Iraq, fighting the war on terrorism, and “dealing with China as a growing superpower” were the primary concerns of the public.

Trump’s Syria intervention comes at a moment when the U.S. is questioning its role abroad. The president ran on a policy of “America first,” an idea that resonated with many in the public who had soured on the effects of outsourcing and decades-long conflicts in the Middle East. Trump himself seemed at times to sneer at the idea that the U.S. bore a moral obligation unique unto itself; “you think our country is so innocent?” he said, responding to a question about Vladimir Putin’s penchant for eliminating political opponents. Some have speculated that his foreign policy will be more isolationist, but the events of this week cast that into somewhat more doubt.

The U.S. has not always held itself to the standard of the superpower with a collective mindset of “to whom much has been given and much is expected.” In May of 1940, People’s Research Survey asked “would it be wiser for us to join the war if the allies seem to be losing or to stay out in the hope that we can live in peace with the new German empire if Hitler wins?”

Sixty-two percent of Americans answered that they’d prefer to stay out and take their chances with Mr. Hitler.