The war in Afghanistan is the longest in U.S. history. It's lasted over 16 years and in that time America's goals and strategies have changed. Now there's another new plan. President Trump has sent 3,000 more troops to train and assist the Afghan army. But in the Afghan capital you don't have to go far to see the problems. Kabul is so dangerous, American diplomats and soldiers are not allowed to use the roads. As we first reported earlier this year, they can't drive just two miles from the airport to U.S. headquarters. They have to fly. After all these years, a trillion dollars, and 2,400 American lives -- Kabul is under siege.

This is rush hour at Kabul International Airport -- a swarm of helicopters that's earned the nickname 'Embassy Air.' It's how Americans and their allies working at the U.S. Embassy and military headquarters travel back and forth from the airport. It's just a five-minute flight. The chopper we boarded was making its tenth trip of the day.

Helicopters await passengers at Kabul International Airport. The danger of the capital has made helicopter rides the safest way to and from the airport. CBS News

A few years ago American convoys regularly drove on the airport road below. Now the view from the helicopter window is all most on board will see of Kabul. They'll stay behind blast walls for the rest of their time in Afghanistan. We wanted to know what it says about where we are in this war if American troops can't drive two miles down a road in Kabul.

John Nicholson: It's a country at war. And it's a capital that is under attack by a determined enemy.

"The war is changing from a war against armies to a war against people."

No U.S. General has spent more time here than John Nicholson -- the commander of American forces in Afghanistan.

John Nicholson: We do everything possible to protect our forces. So…

Lara Logan: You're not using the roads.

John Nicholson: Protecting the lives of our troops is our number-one priority. If we can fly instead of drive and that offers them a greater degree of safety, then it's the prudent and the right thing to do.

Lara Logan: In military terms, that's called surrendering the terrain.

John Nicholson: I disagree. I think it's answering our moral imperative to protect the lives of our soldiers and civilians. So that's what we do.

John Nicholson, commander of American forces in Afghanistan CBS News

But this isn't some remote outpost -- it's the capital. When the U.S. first came here, the population was 500,000. Now it's more than 5 million. Refugees, people desperate for work, and terrorists have flooded Kabul. General Nicholson showed us how vulnerable the city has become.

John Nicholson: A suicide bomber is gonna go in here, he's gonna kill himself. He doesn't care about his future. Vastly easier than what the Afghan security forces have to do.

Lara Logan: Because he doesn't have to have an exit strategy.

John Nicholson: Exactly.

Lara Logan: How easy is it to infiltrate the city, especially one this big?

John Nicholson: Yeah, right now it's easier than we would like.

General Nicholson took command in 2016 shortly after the U.S. cut troop levels to fewer than 10,000. The enemy filled the vacuum. Suicide bombers have terrorized Kabul ever since, shattering police stations, mosques, and foreign embassies. This truck bomb last year killed 150 people. It was the deadliest attack in the capital since the start of the war.

Ashraf Ghani: The level of brutality, the level of heartlessness is unbelievable, and we have to muster all of our resources to be able to deal with this.

Afghan President Ashraf Ghani rules from the presidential palace that's occupied the city center for more than a century. We noticed the walls around him and the rest of the city have expanded and grown taller since our last visit three years ago. Some of the streets we traveled turned into tight corridors of 20-foot high concrete barriers. It made it hard to tell where we were.

Lara Logan: Parts of this city now are unrecognizable. What happened here?

Ashraf Ghani: The war is changing from a war against armies to a war against people.

Lara Logan: More civilians are dying in Kabul every year. And your response is more walls.

Ashraf Ghani: 21 international terrorist groups are operating in this country. Dozens of suicide bombers are being sent. There are factories producing suicide bombers. We are under siege.

By terrorizing the people the Taliban have sown deep doubts about the government. The result: angry protestors in the capital chanting "death to Ashraf Ghani."

Lara Logan: If you can't secure the capital, how are you going to secure the rest of the country?

Ashraf Ghani: You tell me. Can you prevent the attack on New York? Can you prevent the attack on London?

Lara Logan: We're not talking about one attack. A series of attacks right here on your doorstep, a bomb that blew out the windows in your palace that has turned this city into something of a concrete prison.

Ashraf Ghani: What do you want? What's your alternative, ma'am?

Lara Logan: What is the alternative?

Ashraf Ghani: The alternative is resolve.

Afghan President Ashraf Ghani CBS News

Resolve has come at a heavy cost. In just four months last year, more than 4,000 Afghan soldiers and police were wounded, another 2,500 killed. Since then, Ghani has refused to reveal casualty figures. As you will see, it is a sensitive subject.

Lara Logan: Your soldiers and your policemen are dying in unprecedented numbers.

Ashraf Ghani: Indeed.

Lara Logan: How long can that be sustained?

Ashraf Ghani: Until we secure Afghanistan.

Lara Logan: How long is that? How long until you secure it?

Ashraf Ghani: As long as it takes. Generations if need be!

Lara Logan: The U.S. isn't going to be here for generations.

Ashraf Ghani: We will be here for generations. We do not need others to fight our fights.

Lara Logan: People in this country say that if the U.S. pulled out, your government would collapse in three days.

Ashraf Ghani: From the resource perspective they are absolutely right. We will not be able to support our army for six months without U.S. support, and U.S. capabilities.

Lara Logan: Did you just say that without the US support your army couldn't last six months?

Ashraf Ghani: Yes. Because we don't have the money.

American taxpayers bankroll 90 percent of Afghanistan's defense budget. That's more than $4 billion a year. Another $30 billion has been spent rebuilding this country. A bustling city has risen from the ruins. But in all the years we have been coming here, it's never been this dangerous. Checkpoints choke the traffic all over Kabul. It was as difficult to film as it was to move. Terrorists can strike at any time. Nobody knows that better than the men of this elite counter-terrorism unit. They rush to the scene of every attack – such as this one at a Kabul mosque – where a suicide bomber blew himself up just steps away. They took us beyond the barbed wire to the main military hospital -- the site of a chilling attack last year by the Islamic State, one of the many terror groups with a foothold in Kabul.

Lara Logan: The terrorists, they wore the white coats, like a doctor. Right?

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We were told by commanders who were here that five terrorists disguised as doctors got past the hospital's heavy security. They were armed with assault rifles and a weapon that allowed them to quietly move from room to room.

(Former Lieutenant): They had knives. They killed a lot of people with that knife.

Lara Logan: So they were stabbing people in their beds? Stabbing patients?

(Former Lieutenant): Stabbing patient in their beds. Yeah. And opening their stomachs.

This former lieutenant led the assault force that stormed the building. We agreed to conceal his identity to protect him from reprisals.

(Former Lieutenant): They are very clever. And they can do anything inside. They get into the buildings and they start shooting around and show the weakness of the government.

Reinforcements landed on the roof. On the ledges below you can see hospital workers hiding. When cornered, the terrorists detonated grenades strapped to their chests. They murdered more than 50 people that day. Afghans normally bury their dead in a simple cloth shroud. That's not possible when bodies are obliterated by suicide bombers. It happens so often now, Kabul's carpenters have turned to something new -- making coffins. There's also greater demand for prosthetic limbs. This orthopedic clinic is run by the International Committee of the Red Cross.

Lara Logan: You said the security situation is not getting any better.

Dr. Alberto Cairo: Definitely not. I don't see any improvement.

Dr. Alberto Cairo has worked at the clinic for 27 years. He told us he's treating more and more victims of terror attacks.

Lara Logan: So, you know, many people far away from here think this war is over.

Dr. Alberto Cairo: What? The war is over. Please. How can they think of anything like this? No. The war is going on. People are very desperate. People are, they have lost hope.

Lara Logan: Why do you say people have lost hope?

Dr. Alberto Cairo: If you consider that the lifespan of the people in Afghanistan is around 60 years, it means that at least two thirds of them have seen only war. War, war, war.

Dr. Alberto Cairo, who works at a clinic in Kabul CBS News

With America's new strategy, more troops are in, time limits are out, and Pakistan is under pressure for being a safe haven for terrorists. General John Nicholson believes this will end the war, something we've heard from previous commanders.

Lara Logan: Do you have everything you need?

John Nicholson: Yeah, with the new policy I do.

Lara Logan: This is it, right? I mean, there's no more? This is the end game?

John Nicholson: Yes, this is the end game. This is a policy that can deliver a win.

Nicholson is targeting Taliban leaders. This car carried one of their high-ranking commanders. And striking their largest source of revenue, the drug labs that turn Afghanistan's most common crop -- opium --- into heroin. The goal is to do what his predecessors have repeatedly tried and failed -- force the Taliban to cut a deal.

Lara Logan: In 16 years, not a single Taliban fighter has renounced al-Qaeda or embraced, publicly embraced the Afghan constitution. Not a single one.

John Nicholson: In private they do.

Lara Logan: They don't do it publicly.

John Nicholson: But they do it in private.

Lara Logan: It says it all that they won't do it publicly.

John Nicholson: I agree with you.

Lara Logan: Right. So, why all these years people have been trying to bring the Taliban to the negotiating table, they've never come?

John Nicholson: I believe it's because they thought they could win. Because they believed we had lost our will to win. Because since 2009 when we announced the surge, we also announced our exit date. And, so, why, if your enemy has announced when he's leaving, then why would you negotiate?

Lara Logan: All of these people assisted Osama bin Laden and al-Qaeda and we're now saying to the American people, "We can't defeat them, so we're gonna negotiate and put them in the government."

John Nicholson: No, we're killing them in large numbers. They can either lay down their weapons and rejoin society and be a part of the future of Afghanistan, have a better life for their children and themselves, or they can die.

Lara Logan: You know, many Americans look at this and they say, "You know, we've been there 16 years. It's enough now. We should just come home."

John Nicholson: Our country hasn't been attacked in those 16 years. They haven't been attacked from Afghanistan.

Lara Logan: A lotta people at home just don't buy that terrorists are coming from Afghanistan to attack them at home. They're worrying about the guy going to rent a truck from Home Depot and drive into a crowd of civilians.

John Nicholson: Well this raises the point. We need to defeat the ideology. If we were to lose here or if we were to leave here, the cost would be unacceptable. Why? It would embolden jihadists globally, those living in our own countries. It would convince them of the ultimate success of their cause. In my view the cost of failure here is unacceptable.

General John Nicholson told us he's giving himself two years to deliver major changes. But it's hard not to be skeptical in a city where the enemy has driven American forces from the roads -- into the sky.

Since we first broadcast our story, General John Nicholson has made securing the capital a priority. He's ordered more special operations missions inside Kabul to target the Taliban and terrorist networks attacking the city.

Produced by Guy Campanile, Richard Butler, Andrew Bast, and Ahmad Mukhtar.