Former Republican Sen. Chuck Hagel of Nebraska says uprisings in the Arab world are part of “a global rebalancing” in which the international clout of the United States will be diminished.

The war in Afghanistan — the longest in U.S. history — has become “unsustainable,” now that most people think the troops should come home, former U.S. Sen. Chuck Hagel told the World Affairs Council of Pittsburgh on Wednesday.

Hagel, a Republican who served as Nebraska’s senator from 1997 to 2009 and became an authority on U.S. foreign policy, spoke in advance of President Obama’s speech on the country’s Middle East policy, which is scheduled for today.

A Gallup poll conducted May 5 to 8, the week after Navy SEALs killed Osama bin Laden, found nearly 60 percent of people think the United States has accomplished its mission in Afghanistan and should bring its troops home.

“So you already know that that’s unsustainable,” Hagel told about 80 people at the Duquesne Club, Downtown. The debate now “is about when we come out, and how we come out. … You can’t keep the troops in there and prop up a government.”

Hagel, a Vietnam veteran and former member of the Senate Foreign Relations Committee, took part in Senate debates about authorizing the use of force in Afghanistan, and said the mission then centered on getting bin Laden.

“We never even talked about nation-building,” Hagel said.

A bigger concern than Afghanistan is its neighbor, Pakistan, a nuclear-armed country with a nuclear-armed enemy in India, bordered by a nuclear-armed China. Pakistan occupies “the most dangerous corner of the world,” Hagel said.

He said he’s certain some Pakistani military and intelligence officials knew where bin Laden was hiding, but said he doesn’t believe the country’s top leaders were in on it.

The revolutions sweeping North Africa and the Middle East aren’t driven by political ideology or even a yearning for democracy, Hagel said.

“When this young fruit vendor sets himself on fire (in Tunisia), do you think this was about some political doctrine? Do you think this was about democracy? No, this was about basic dignity, basic survival, about having a chance to live,” he said.

Hagel said the so-called Arab Spring is part of a “global rebalancing,” aided by technology and communication, that’s sapping the United States’ authority over the rest of the world. That’s not necessarily a bad thing, Hagel said, because it’s driven by economic opportunity in parts of the world that sorely need it.

White House officials said yesterday that Obama plans to propose in his speech today making billions of dollars in economic assistance available, mostly to foster private-sector growth in Egypt and Tunisia.

Hagel pointed to countries like Brazil and Turkey, which once depended on the United States, but now have growing economic and strategic importance.

“There was a time when they pretty much did what we told them,” he said. “That’s not the way it works any more.”