The United States has been at war continuously since the attacks of 9/11 and now has just over 240,000 active-duty and reserve troops in at least 172 countries and territories. While the number of men and women deployed overseas has shrunk considerably over the past 60 years, the military’s reach has not. American forces are actively engaged not only in the conflicts in Afghanistan, Iraq, Syria and Yemen that have dominated the news, but also in Niger and Somalia, both recently the scene of deadly attacks, as well as Jordan, Thailand and elsewhere.

An additional 37,813 troops serve on presumably secret assignment in places listed simply as “unknown.” The Pentagon provided no further explanation.

There are traditional deployments in Japan (39,980 troops) and South Korea (23,591) to defend against North Korea and China, if needed, along with 36,034 troops in Germany, 8,286 in Britain and 1,364 in Turkey — all NATO allies. There are 6,524 troops in Bahrain and 3,055 in Qatar, where the United States has naval bases.

America’s operations in conflict zones like Africa are expanding: 400 American Special Forces personnel in Somalia train local troops fighting the Shabab Islamist group, providing intelligence and sometimes going into battle with them. One member of the Navy SEALs was killed there in a mission in May. On Oct. 14, a massive attack widely attributed to the Shabab on a Mogadishu street killed more than 270 people, which would show the group’s increased reach. About 800 troops are based in Niger, where four Green Berets died on Oct. 4.

Many of these forces are engaged in counterterrorism operations — against the Taliban in Afghanistan, for instance; against the Islamic State in Iraq and Syria; against an affiliate of Al Qaeda in Yemen. So far, Americans seem to accept that these missions and the deployments they require will continue indefinitely. Still, it’s a very real question whether, in addition to endorsing these commitments, which have cost trillions of dollars and many lives over 16 years, they will embrace new entanglements of the sort President Trump has seemed to portend with his rash threats and questionable decisions on North Korea and Iran.

Andrew Bacevich, a retired Army colonel who lost a son in Iraq and is a critic of military operations, says that “a collective indifference to war has become an emblem of contemporary America.” The idea that Americans could be inured to war and all its horrors is chilling, and it’s a recipe for dangerous decisions with far-reaching ramifications. There are many factors contributing to this trend:

During earlier wars, including Vietnam, the draft put most families at risk of having a loved one go to war, but now America has all-volunteer armed forces. Less than 1 percent of the population now serves in the military, compared with more than 12 percent in World War II. Most people simply do not have a family member in harm’s way.

American casualty rates have been relatively low, especially in more recent years after the bulk of American troops were withdrawn from Afghanistan and Iraq. Also, the United States has shifted to a strategy in which Americans provide air power and intelligence, and train and assist local troops who then do most of the fighting and most of the dying. This year, for instance, 11 American service members died in Afghanistan and 14 in Iraq. By comparison, 6,785 Afghan security force members died in 2016 and 2,531 died in the first five months this year, according to the United States and Afghan governments. Tens of thousands of civilians also perished at the hands of various combatants, including in 2017, but the figures get little publicity. Most Americans tend not to think about them.

Since 9/11, American leaders have defined the fight against terrorism as a permanent struggle against a permanent threat. Mr. Obama withdrew significant forces from Afghanistan and Iraq. But the rise of ISIS in Iraq and Syria and a resurgent Taliban in Afghanistan led to renewed engagement, though at lower troop levels. Terror attacks here and in Europe, and Mr. Trump’s scaremongering, have reinforced the public’s sense of siege.

The military is essential to national security, but it is not the only thing keeping America safe. So do robust diplomacy and America’s engagement in multilateral institutions, both of which we have faulted Mr. Trump for ignoring or undercutting. The Pentagon, by contrast, thrives. After some belt-tightening during the financial crisis, it has a receptive audience in Congress and the White House as it pushes for more money to improve readiness and modernize weapons. Senators who balk at paying for health care and the basic diplomatic missions of the State Department approved a $700 billion defense budget for 2017-18, far more than Mr. Trump even requested.

Whether this largess will continue is unclear. But the larger question involves the American public and how many new military adventures, if any, it is prepared to tolerate.