WASHINGTON — President Obama’s defense secretary, Ashton B. Carter, has become the secretary of reassurance.
In April, Mr. Carter was in Tokyo, reassuring officials worried about China that the United States would back Japan’s administrative control over disputed islands in the East China Sea and provide its advanced F-35 fighter jets to its foremost Pacific ally.
In June, he was in Tallinn, Estonia, reassuring officials worried about Russia that the United States would place battle tanks, infantry fighting vehicles and other heavy weapons in Baltic and Eastern European states.
And in July, he was standing on the hill of a remote outpost on Israel’s northern border with Lebanon, reassuring officials worried about Iran that despite the pending nuclear deal with Tehran, which the Israeli prime minister opposes, the United States would further increase military cooperation with Israel, including providing it with F-35 fighters.
Mr. Carter has adopted this diplomatic role since taking the reins of the Pentagon from Chuck Hagel in February. The confluence of turmoil around the world has American allies looking to the United States for promises of military support if security conditions worsen.
In Eastern Europe, Russia’s foray into Ukraine has set off fears of another Cold War. In Asia, a rising and more aggressive China has alarmed America’s Pacific allies. And in the Middle East, Sunni Arab states and Israel have formed a de facto alliance of concern over what they view as a strengthened Iran.
The pleas are being made of a president, Mr. Obama, who is loath to commit American military troops to foreign wars. So the administration’s answer to such requests has been to deploy Mr. Carter.
Secretary of State John Kerry is on his own reassurance trip this week. On Monday, he was in Doha, Qatar, meeting Sunni Arab foreign ministers who have concerns about the Iran nuclear deal. But as the main negotiator of the pact, Mr. Kerry has also assumed more of the bad cop role to Mr. Carter’s good cop.
Officials particularly in Israel are wary of Mr. Kerry for negotiating an agreement they do not like, finding it easier to turn to Mr. Carter for promises that the American military will continue to support them.
By sending the defense secretary to calm fretful allies, the administration can give the appearance of robust military support without actually committing American troops.
“There is a huge demand for American leadership, especially as measured by military activity and presence,” said Derek Chollet, a former assistant secretary of defense. “But of course, we can’t do everything everywhere simultaneously.”
For the administration, added Mr. Chollet, who is now a senior adviser for security and defense policy at the German Marshall Fund, “the challenge is how to manage the trade-offs.”
So far, meeting that challenge has consisted of sending Mr. Carter around the world to tell allies that he has their back, even if his boss will not be sending any troops. Instead, the Carter message — a direct echo of Mr. Obama’s own views — has been to encourage allies to build up their own defenses, with help from American trainers, military equipment and weaponry.
Of those allies, the Sunni Arab states have been the most assertive. The Saudi-led campaign in Yemen, for example, has prompted the Sunni Persian Gulf countries to engage militarily as never before.
The Saudis have assembled a group of Sunni nations to attack Houthi militia fighters who took over Yemen’s capital, Sana, and ousted a government backed by Saudi Arabia and the United States. Saudi officials have said that the Houthis are being covertly backed by Shiite-dominated Iran.
Other nations that have joined the coalition against the Houthis, like Morocco, have characterized their participation in blunt sectarian terms.
The United States has not sent troops to Yemen or engaged in airstrikes there, and in fact Obama administration officials have been prodding the warring sides to come to a political settlement. But the United States has also backed the Saudi campaign with additional weaponry, reconnaissance and intelligence.
Mr. Carter has spoken often of Iran’s “malign influence” in Yemen and other areas in the region as part of his campaign to reassure Arab allies that the United States has their back — while at the same time encouraging those countries to fight their own battles.
Similarly, the Defense Department has been pushing Japan to adopt a more aggressive military posture, after decades of relying on the United States for security.
In April, just after Mr. Carter’s trip to Tokyo, the United States and Japan announced an agreement in Washington to expand the reach of Japan’s military, previously limited to its own defense. It allows Japan to act militarily if the United States or countries American forces are defending are threatened. The agreement reflected worries about China, whose territorial claims in the South China and East China Seas and growing military spending have upset its neighbors.
In his good-cop role, Mr. Carter has been taking pains to publicly express that he understands, and cares about, the security needs of America’s allies.
Surveying Israel’s border with Lebanon two weeks ago from a military post known as Hussein Lookout, Mr. Carter appeared grave as Israeli officials told him that Iranian-backed Hezbollah forces were embedded in civilian populations in the hills beyond.
“Probably Hezbollah is listening right now,” the Israeli defense minister, Moshe Ya’alon, joked to Mr. Carter.
“Hezbollah is sponsored of course by Iran,” Mr. Carter said afterward. “We will continue to help Israel counter Iran’s malign influence.”
Because of an editing error, an earlier version of a picture caption with this article misstated where Defense Secretary Ashton B. Carter was when the photograph was taken last month. It was Israel, not Jordan.