(CNN) -- Afghan forces formally took over security responsibilities for their violence-plagued country from NATO-led troops on Tuesday, marking a key transition in the long and costly war.
"You are the sons and guardians of this country, and it is your responsibility to protect it," Afghan President Hamid Karzai told his troops at a handover ceremony in Kabul. "I wish a long-term peace in Afghanistan."
The head of NATO said Afghans are now in charge.
"The main effort of our forces is shifting from combat to support. We will continue to help Afghan troops in operations, if needed, but we will no longer plan, execute or lead these operations," NATO secretary-general Anders Fogh Rasmussen said at the ceremony.
"By the end of 2014, our combat mission will be completed," Rasmussen said.
With the handover, U.S. and coalition forces move into a backup role and let Afghan National Security Forces -- long questioned for their preparedness and commitment -- handle the combat in the restive nation.
British Defense Minister Philip Hammond called the handover a "hard-fought milestone."
Afghanistan will continue to face challenges as it builds toward becoming a secure and stable state," he said. "The Afghanistan our combat forces leave at the end of 2014 will not be perfect, but will be able to stand independently and will never again provide a haven for terrorists to attack the West."
During his State of the Union address in January, President Barack Obama said, of the approximately 66,000 U.S. troops in Afghanistan, more than half -- 34,000 -- will come home in the next year and the country's war in Afghanistan would be over by the end of 2014.
The Taliban regime in Afghanistan was sheltering the al Qaeda terror network when it launched attacks against the United States on September 11, 2001. The next month, the United States cranked up military operations that led to the toppling of the Taliban government.
Ever since, international forces have been fighting radical Islamic militants in Afghanistan and neighboring Pakistan.
What does this news mean for Afghanistan and the United States? Here are some key questions that will be asked in the coming months:
1. Are the Afghan troops up to the task?
There are certainly doubts.
A Pentagon review in December claimed that only one of 23 Afghan army brigades was capable of functioning on its own.
Meanwhile, literacy rates are low, desertion rates are high, and many deserters have joined the insurgency. There also have been a troubling number of "green-on-blue" attacks: Afghan troops attacking their American comrades.
But then-Defense Secretary Leon Panetta spoke positively about the progress Afghans had made in growing their army, reducing violence and becoming more self-sufficient. At the time, Afghan forces were leading nearly 90% of operations across the country.
"We're on the right path to give (Afghanistan) the opportunity to govern itself," Panetta said.
Karzai has said he welcomes the U.S. troop withdrawal and insists his army can defend the country against the Taliban.
"It is exactly our job to deal with it, and we are capable of dealing with it," Karzai said during an interview with CNN's Christiane Amanpour.
2. What are the biggest challenges?
The main fear among Afghans is that the country could revert to another civil war once the United States withdraws its combat troops.
"Some people we've spoken to sort of take it for granted that there's going to be a civil war when the United States leaves," said CNN's Erin Burnett on a trip last year to Afghanistan. "It happened before when the Soviet Union left (in 1989)."
Above all, Karzai said the Afghan army needs the tools to battle the insurgents, namely more equipment and firepower. He came to the Pentagon in January with a wish list asking for more helicopters, drones and other hardware, according to a senior defense official.
"We need an air force. We need air mobility," Karzai told Amanpour. "We need proper mechanized forces. We need, you know, armored vehicles and tanks and all that."
Retired Army Gen. Stanley McChrystal, once America's top commander in Afghanistan, said the Afghan people are "terrified."
"They're terrified because they think they have something to lose," McChrystal said. "There has been progress made. There is a better life. There are girls in school. There are things that are better than they were and opportunities potentially ahead.
"But they're afraid that if we completely abandon them in 2014, as they perceive we did in 1989, (things) would all go back."
And in Washington, there are worries that the wrong move could put the United States right back where it started, with nothing to show for a bloody conflict that started in 2001.
3. How big a threat does the Taliban still pose?
The Taliban are still "resilient and determined," according to a recent Pentagon report, and pose a major security threat.
The Taliban continues to carry out high-profile attacks in the capital, Kabul, even targeting the