Thirteen years after the Sept. 11 terror attacks, this was supposed to be a season of relief, with Iraq managing on its own and most U.S. troops finally ending their combat duty in Afghanistan. Instead, Americans are bracing for another upsurge of military engagement in a region where one war blurs into another. Across the world, a generation has now grown up amid this continuous conflict, and there's no end in sight.

"The Cold War took 45 years," said Elliott Abrams, a longtime diplomat who was top Middle East adviser to President George W. Bush. "It's certainly plausible that this could be the same. ... It's harder to see how this ends."

For now, President Barack Obama seems to have bipartisan support as he prepares to outline his plans Wednesday for expanded operations against militants of the so-called Islamic State who have overrun large swaths of Iraq. His administration has cautioned that the effort could take several years.

Short-term, Obama has public opinion with him; a new Washington Post-ABC News poll found 71 percent of Americans supporting airstrikes against the Islamic State fighters, compared to 45 percent in June. Longer-term, a Pew Research Center-USA Today poll last month suggested that most Americans view the world as becoming more dangerous and expect militant forms of Islam to grow in influence rather than subside.

Since the autumn of 2001, America, with its allies, has been at war against factions of Islamic militants and terrorists, including the Taliban and al-Qaida, as well as offshoots in Yemen, Somalia and elsewhere.

Indeed, some analysts say the conflict dates back further, citing such incidents as the 1993 bombing of the World Trade Center in New York and the 1983 bombing that killed 241 U.S. servicemen at a barracks in Lebanon. Military historian Max Boot suggests the starting point was the Iranian revolution of 1979, when the U.S. Embassy in Tehran was seized and its staff held hostage for 444 days.

"For the first time, we understood the threat by armed Islamist extremism," said Boot, a senior fellow at the Council on Foreign Relations and former adviser to Republican presidential campaigns. "We didn't face up to it — we tried to ignore it as long as possible. But after 9/11, we couldn't ignore it anymore."

The Sept. 11 attacks triggered the invasion of Afghanistan by the U.S. and its allies, starting in October 2001, with the aim of dismantling al-Qaida's base of operations and toppling the Taliban regime. The Taliban, though quickly ousted from power, has been waging an insurgency ever since.

In 2003, the U.S. spearheaded an invasion of Iraq, citing various justifications but nonetheless categorizing the conflict as part of "the Global War on Terrorism." Iraqi dictator Saddam Hussein was captured, tried and executed, yet an insurgency arose against the U.S.-led coalition waged by various factions, including al-Qaida affiliates and Sunni militants who were precursors of the Islamic State group.

Obama's plans for an expanded mission against Islamic State fighters are expected to include intensified airstrikes but no major deployment of ground troops, along with a heavy reliance on allies. The role of Middle East nations could be pivotal, said Wathiq al-Hashimi, director of the al-Nahrein Center for Strategic Studies in Baghdad.

"The United States failed in both Afghanistan and Iraq, but this time round may be different since the Islamic State is posing a serious danger to close U.S. allies in the region who cannot defend themselves on their own," al-Hashimi said. "The United States will be going in this time with the blessing of regional powers."

Ibrahim Hooper, spokesman for the Washington-based Council on American-Islamic Relations, contends that much of the Middle East's conflicts could have been avoided or eased if the U.S. government had been less willing to tolerate authoritarian regimes and more willing to criticize Israel's policies toward the Palestinians.

Hooper said the Islamic State group's ascension in Iraq could have been prevented if the U.S. had insisted on a nonsectarian Iraqi government, rather than the one led by recently replaced Prime Minister Nouri al-Maliki that favored Shiite Muslims over the Sunnis. Similarly, Hooper said the U.S. could have deprived Islamic State of its strongholds in Syria by intervening early in Syria's civil war on behalf of moderate rebels opposing President Bashar Assad.

"Our counterproductive policies have created a political vacuum in which ISIS can flourish," said Hooper, using an acronym for the Islamic State group. "Without massive injustices in the region, they would not exist."

James Jay Carafano, a national security expert with the conservative Heritage Foundation, offered a contrasting analysis, blaming Obama for "taking his foot off the pedal" by withdrawing U.S. troops from Iraq in 2011 and thereby emboldening Islamic State fighters.

Back in 2003, Carafano published a commentary titled, "The Long War Against Terrorism" in which he urged Americans to brace for a sustained struggle.

"Such a war requires our leaders to understand that our staying power, our will to win, is as important as any weapon in our arsenal," he wrote.

However weary of war, the American public is willing to back aggressive, long-term engagements overseas, Carafano argued in a telephone interview this week.

"All our conflicts start out popular, but only World War II stayed that way," Carafano said. "People gradually get less excited over time.

"But Americans are relatively practical people," he added. "If you're doing the right thing and it's working, they'll be with you."

Looking ahead, experts familiar with the Middle East say it's hard to foresee a total victory for the U.S. and its allies any time soon. Elliott Abrams, for example, noted that many hundreds of young people from the West were eager to join the Islamic State group, enabling it to replenish its ranks and gird for a long struggle.

"It's clear that the Americans have made up their minds to get involved in what is likely to be an open-ended war," said Hilal Khashan, professor of political science at the American University of Beirut. "The Americans know how to start a war, but not to end one."

"The Americans' intervention is selective: They invaded Iraq but left Iran alone, they are leaving Israel to do as it pleases in Gaza, they are leaving the Syrian regime to kill its people," Khashan added. "And whenever they intervene, they just make things worse. They may destroy the Islamic State, but what happens to the problems in Iraq and Syria?"

Daniel Byman, research director at the Brookings Institution's Center for Middle East Policy, said he prefers the term "manageable" over "winnable" as a goal in trying to counter threats from the region's extremists and terrorists.

"There's no clear victory point when the enemy gives up," Byman said. "There's likely to be some level of terrorism, but it can't be to the point where it disrupts our lives in some fundamental way."

Max Boot suggested the overall conflict was winnable — but only through a long-term struggle comparable to the Cold War.

"This radical, armed Islamism will burn itself out," he said. "The problem is an awful lot of people who will die between now and then."