President Obama on Wednesday charted a muscular new course for the United States in a turbulent world, telling the United Nations General Assembly in a bluntly worded speech that the American military would work with allies to dismantle the Islamic State’s “network of death” and warning Russia that it would pay for its bullying of Ukraine.
Two days after ordering airstrikes on dozens of militant targets in Syria, Mr. Obama issued a fervent call to arms against the Islamic State — the once-reluctant warrior now apparently resolved to waging a twilight struggle against Islamic extremism for the remainder of his presidency.
“Today, I ask the world to join in this effort,” Mr. Obama said, seeking to buttress a global coalition that he said would train and equip troops to fight the group, also known as ISIL, starve it of financial resources, and halt the flow of foreign recruits to its ranks.
“Those who have joined ISIL should leave the battlefield while they can,” Mr. Obama said, foreshadowing the blows to come. “For we will not succumb to threats, and we will demonstrate that the future belongs to those who build, not those who destroy.” The brutality of the militants, he said, “forces us to look into the heart of darkness.”
Even so, Mr. Obama said, the threat from the Islamic State was only the most urgent of an onslaught of global challenges that have given the United States no choice but to take the lead: from resisting Russia’s aggression against Ukraine to coordinating a response to the Ebola outbreak in West Africa; from brokering a new unity government in Afghanistan to organizing a new campaign to confront climate change.
It was a starkly different president from the one who addressed skeptical world leaders at the General Assembly last year, two weeks after calling off a missile strike on Syria over its use of chemical weapons. In that speech, Mr. Obama offered a shrunken list of American priorities in the Middle East and showed little appetite for the charged rhetoric or interventionist policies of his predecessor, George W. Bush.
Mr. Obama on Wednesday spoke more like a wartime leader, reaffirming his determination to work with other countries but leaving little doubt that the United States would act as the ultimate guarantor of an international order that he said was under acute stress.
As if to underscore his new role, Mr. Obama headed a rare leaders session of the United Nations Security Council, which unanimously passed a resolution requiring countries to pass laws against traveling abroad to join terrorist groups or financing those efforts.
“If there was ever a challenge in our interconnected world that cannot be met by one nation alone, it is this,” he said, “terrorists crossing borders and threatening to unleash unspeakable violence.”
For all the hardening of Mr. Obama’s tone, though, it remained unclear whether the speech represented a fundamental rethinking of his policy or a reluctant response to the threat posed by the Islamic State, brought home for many Americans after the militants posted Internet videos of American hostages who were beheaded.
The strategy he outlined would protect the United States from terrorist threats by crippling the Islamic State and other militants like the Khorasan Group, which was also targeted this week by American airstrikes, and not by trying to transform the societies in which they took root, as did the architects of the Iraq war.
Still, his remarks clearly seemed intended to get past months in which the president appeared visibly conflicted about the proper use of American military force in the Middle East — an ambivalence that opened him to criticism that he was feckless and irresolute.
In addressing the Ukraine crisis, Mr. Obama used his strongest language yet, portraying Russia’s incursions as an affront to the principles of the United Nations and promising to levy a cost on President Vladimir V. Putin. He accused Russia of conspiring with Ukrainian separatists to obstruct aninvestigation into a downed Malaysian jetliner.
“This is a vision of the world in which might makes right,” Mr. Obama said, “a world in which one nation’s borders can be redrawn by another, and civilized people are not allowed to recover the remains of their loved ones because of the truth that might be revealed.”
The 39-minute speech was also notable for what he did not say. Last year, he singled out nuclear negotiations with Iran and Syria’s civil war as two of his top priorities in the Middle East. On Wednesday, he mentioned them in only a cursory manner.
Iran, he said, should not let the chance for a nuclear agreement slip by. But he made no reference to Iran’s president, Hassan Rouhani, who has made clear he does not want to shake hands with Mr. Obama this week, a gesture long-awaited as a symbol of thawed relations between Iran and the United States. Privately, American officials have expressed deep skepticism about the status of the negotiations with Tehran, and Mr. Obama’s subdued remarks suggested he shares that pessimism.