Two years after the United States withdrew all combat troops from Iraq, the U.S. military is helping Iraq again in the fledgling government's all-out battle against insurgents.

A massive C-17 cargo plane landed in Baghdad on a recent evening to deliver ammunition - this time, 2,400 rockets - for Iraqi helicopters, military officials said, as Iraqis fight a resurgent al Qaeda in al Anbar Province

Related: US responds as al Qaeda overruns Fallujah.

Fallujah has once again become a familiar battleground. It was in Fallujah in November 2004 that U.S. Marines fought a ferocious, house-to-house battle against Iraqi insurgents. More than 100 Marines died to pacify the city and hundreds more were injured.

It's been two years since America ended its combat role in Iraq after the ouster of Saddam Hussein. With U.S. forces gone, al Qaeda has once again taken control of Fallujah.

This time, it was after Iraqi Prime Minister Nouri al Maliki pulled the Iraqi military out of the city and turned security control of the city over to Iraqi police. Several hundred al Qaeda fighters took advantage of the security vacuum and took over the city.

Related: US Gen. Ray Odierno says Fallujah is "disappointing"

U.S. Ambassador to Iraq Robert Beecroft told ABC News that al Qaeda posed a threat to all of the country.

The terror group has been responsible for deadly attacks that, according to the United Nations, killed 7,818 people last year, the highest number in years.

In Baghdad alone, bombings have killed more than 700 people in the last month - more than double last year's number.

Iraqi and American officials say al Qaeda is using the terror attacks in hopes of reigniting the sectarian battles between Shiite and Sunni Muslims that drove Iraq into civil war a few years ago.

"It's a very precarious situation," said Beecroft. "They're capable of serious assaults and they're capable of taking and holding ground and causing a lot of trouble."

Beecroft estimates there are 2,000 al Qaeda fighters throughout Iraq.

Two years ago, just before U.S. combat forces pulled out of Iraq, Americans could walk around Fallujah. Today, the conditions are very different.

In Fallujah, al Qaeda is winning.

Iraqi forces now surround the city with checkpoints and armored vehicles. About five miles from Fallujah, the roads are nearly deserted. Iraqi forces warned that it was too dangerous to go any closer to the city.

Beecroft said the U.S. is encouraging the Iraqi government to work with Sunni tribesmen in the Fallujah area so they can do the "heavy lifting" and that a military assault should only be used as a last resort.

To help Iraqi forces with a fight for Fallujah, the U.S. is meeting additional arms requests that Iraq has made on short notice, officials said.

In December, 75 Hellfire missiles were provided as part of the Foreign Military Sales program and an additional 100 Hellfire missiles, 10 Scan Eagle drones and 48 Raven drones will be delivered later this year, officials have said.

Beecroft - who lives with 5,500 other Americans in the world's largest and most heavily fortified embassy - said Americans back home should be very concerned about what is happening in Iraq.

"It's very much in our interest to have an Iraq that is stable, and not to have sectarian conflict or al Qaeda presence that can be used to destabilize the rest of the region and potentially at some point as a platform to attack targets beyond the region," Beecroft said.

ABC News' Martha Raddatz and Luis Martinez contributed to this story.