One hundred years ago, a pacifist president reluctantly led the United States into World War I. Woodrow Wilson eventually won "the war to end all wars," but failed to achieve his greatest desire: a new world order. That would wait until after World War II. Faint echoes of Wilson sounded throughout President Obama's address to the United Nations on Wednesday.

There stood a man who rose to power by opposing George W. Bush's "dumb war" in Iraq, who slowly and reluctantly recognized the gathering dangers of an Islamic State, now urging the league of nations to build a timeless peace—even as he vowed to shed blood for it.

That dichotomy, part of a nest of dualities embedded in the nation's spirit, came with Obama's first words to the balky U.N. "We come together," he said, "at a crossroads between war and peace; between disorder and integration; between fear and hope."

(Related: Obama Takes on (Most of) the World)

Obama called this the best time to be alive, "yet there is a pervasive unease in our world," because the same forces shrinking the globe also harbor new dangers. Giving three examples—the Ebola outbreak, Russia's invasion of Ukraine, and the rise of ISIS in Iraq and Syria—Obama said a common problem is the failure of institutions to adjust to massive change.

Health care systems are broken.

Nations are ignoring international norms forged after World War II.

Finally, he said, the international community has failed to confront the forces that breed terrorism.

It was perhaps the most thoughtful, grounded, and forward-looking speech of Obama's career. Not because he raised expectations with poetic phrasing—he's been there, done that—but because he didn't go there again. Instead, the president offered listeners a bracing, pragmatic road map to the future—a vision that, when moored to reality, was oddly optimistic.

The world has a choice, Obama said: Work together to tackle the problems of a new age, or be swamped by them. In the United States, he said, "we choose hope over fear"—a line that could have been borrowed not only from Wilson, but from other presidents who presided in times of great inflection: Abraham Lincoln, Franklin Roosevelt, John F. Kennedy, both presidents named Bush.

He challenged the nations' leaders to observe and enforce international norms, singling out Russia's advances, Iran's nuclear program, instability in Asia, and finally, the issues of climate change and extreme poverty. "On issue after issue," Obama said, "we cannot rely on a rule book written for a different century."

And then he turned to terrorism, which he called an old phenomena that has grown exponentially lethal in the 21st century. "Humanity's future may depend" on the ability of nations to unite against sectarian and ideological hatred.

He said that starts, but doesn't end, with degrading and ultimately destroying ISIS. "No God condones this terror. No grievance justifies these actions. There can be no reasoning—no negotiation—with this brand of evil," Obama declared. "The only language understood by killers like this is the language of force."

He warned ISIS fighters to "leave the battlefield while they can."

The president seemed to recognize the incongruousness of the moment. A pacifist talking like a hawk? A peace-loving nation pushing the world to war? That's us, Obama said; America is a dichotomous nation, one that doesn't always live up to its ideals. Yes, there is racial unrest in the "small American city of Ferguson, Missouri," he said, and the United States must find its way amid growing globalization and diversity. "But we welcome the scrutiny of the world," he said, because the United States is constantly evolving and struggling and gaining on its promise "to make our union more perfect."

Obama could have drawn a line to his own presidency and his own failings, and he could have questioned how he must evolve as a leader in his remaining months as president. But he didn't. Instead, the 2009 Nobel Peace Prize winner closed with a thinly veiled warning.

"And at this crossroads, I can promise you that the United States of America will not be distracted or deterred from what must be done. We are heirs to a proud legacy of freedom, and we are prepared to do what is necessary to secure that legacy for generations to come," he said. With the words "whatever is necessary" still echoing in the ears of world leaders, from history's lips to Obama's, he concluded: "Join us in this common mission, for today's children and tomorrow's."