WASHINGTON -  What was President Barack Obama's hardest decision during his two terms in office? A massive troop surge in Afghanistan, he said in an interview aired Sunday.

"Toughest decision was early in my presidency when I ordered 30,000 more troops into Afghanistan," Obama, who leaves office in less than two weeks, told ABC's "This Week."

After a protracted debate inside the administration that pitted US military commanders against White House advisers, Obama announced the 30,000-trooop "surge" in December 2009.

The decision would bring the US contingent in Afghanistan to nearly 100,000 troops along with almost 50,000 allied troops.

"I think it was the right decision because the Taliban at that point had gotten a lot of momentum before I'd gotten into office, partly because we hadn't been paying attention as much as we needed to Afghanistan," Obama said in the interview taped Friday.

The president had hoped to withdraw most US military forces from Afghanistan by now, leaving behind just a small force.

But the United States still has some 8,400 military personnel in the country, and announced Friday it is sending some 300 Marines to Helmand province in the coming months.

Meanwhile in Iraq, more than 5,000 US soldiers are still on the ground providing critical support to the country's army, which is unable to man a war alone against the Islamic State group's extremist fighters.

Asked whether he found it disappointing that so many troops remain in both countries, Obama said the United States is "not going to get the kind of decisive, permanent victories in this fight against terrorism that we would get from fighting another country."

"But we don't have this huge footprint, we are less likely to be targeted as, you know, occupiers," he added about the reduced number of troops.

Even after decimating Al-Qaeda in Pakistan's tribal areas and killing the jihadist group's leader Osama bin Laden, groups in the region still "have both the interest and the capacity if we don't maintain vigilance to strike against the United States," he said.

OBAMA UNDERESTIMATED

IMPACT OF RUSSIAN HACKING

Obama admitted that he "underestimated" the impact misinformation and hacking can have on democracies, following an intelligence report on Russian meddling in the US presidential election.

Obama also warned his Republican successor Donald Trump, who takes office in less than two weeks, about the difference between governing and campaigning, saying the president-elect won't be able to run his presidency "the way you would manage a family business."

The interview took place the same day the US intelligence agencies released an unprecedented report saying Russian President Vladimir Putin ordered a campaign of hacking, leaking, and media manipulation aimed at undermining Democratic presidential candidate Hillary Clinton and boosting Trump.

Although Obama said he was not taken in by Russian hostility to the United States, "I think that I underestimated the degree to which, in this new information age, it is possible for misinformation for cyber hacking and so forth to have an impact on our open societies, our open systems, to insinuate themselves into our democratic practices in ways that I think are accelerating."

Obama ordered the intelligence report, released to the public Friday, partly "to make sure that we understand this is something that Putin has been doing for quite some time in Europe, initially in the former satellite states where there are a lot of Russian speakers, but increasingly in Western democracies."

"We have to pay attention," he said of upcoming elections in European ally nations, warning of possible interference.

Russia initially aimed its unprecedented cyber campaign at damaging a potential Clinton presidency, and then turned to supporting Trump after his victory appeared possible, the report from the Director of National Intelligence said.

Trump, who met the country's leading intelligence agency chiefs to hear the full report, has accepted the possibility of Moscow's involvement in hacking US targets, including the Democratic National Committee, but rejected the conclusion that Russia interfered in the election.

Trump's transition team on Sunday doubled down on its claim that Democrats are to blame for allowing their email accounts to be hacked, flatly rejecting any notion Russia may have affected the election.

"Any attempt, any aspiration to influence our elections failed," Trump's senior aide Kellyanne Conway told CNN.

The president-elect has ridiculed US intelligence agencies over their findings and promised he would establish closer ties to Moscow.

However, Obama urged him "to develop a strong relationship with the intelligence community."

"If we're not vigilant foreign countries can have an impact on the political debate in the United States in ways that might not have been true 10, 20, 30 years ago in part because of the way news is transmitted and in part because so many people are skeptical of mainstream news organizations," he said.

"In that kind of environment, where there's so much skepticism about information that's coming in, we're going to have to spend a lot more time thinking about how do we protect our democratic process."

Obama also emphasized focusing "a lot more" on cyber security.

Obama described his talks with the incoming president as "cordial."

"He has been open to suggestions," he added, describing Trump as "very engaging and gregarious."

"I've enjoyed the conversations that we've had. He is somebody who I think is not lacking in confidence," which is "probably a prerequisite for the job."

However, Obama believes Trump "has not spent a lot of time sweating the details" of policies, something he said could be "both a strength and a weakness."

Obama also weighed in on Trump's constant use of Twitter. "Clearly this worked for him, and it gives him a direct connection to a lot of the people that voted for him," he said.

But when Trump becomes president, he cautioned, "there are world capitals and financial markets and people all around the world who take really seriously what he says, and in a way that's just not true before you're actually sworn in as president."

Obama said his hardest choice as president was his 2009 decision to order 30,000 more troops into Afghanistan.

Asked whether he believes he failed to live up to his promise to change the country's trajectory, Obama said his presidency had inspired a new generation of Americans who value diversity, equality and fairness.

That generation is "not the majority yet, but they're going to be the majority soon," he said.

"Part of what I'm interested in doing after I get out of the presidency is to make sure that I'm working with that next generation so that they understand you can't just rely on inspiration."