Story highlights Defense Authorization Act includes expansion of embassy security forces

The number of Marines guarding U.S. embassies will nearly double

A quick-response unit also will allow for reinforcements when needed

Trainees undergo rigorous training and must attain high security clearance

There aren't many people who would voluntarily be pepper-sprayed, but at Marine Corps Base Quantico in Virginia young Marines are doing just that.

It's part of their training to protect U.S. embassies around the world.

After they get a face full of pepper spray, the Marines must immediately complete a course that includes various defensive obstacles.

"We need to have an understanding of what it does," 21-year-old Cpl. Haley Whisenant explained to CNN about 20 minutes after having been sprayed directly in her eyes. "If we were to spray someone and we are contaminated with it as well, we need to know how to contain ourselves," she said.

The embassy guard training also includes one-on-one defensive tactics and weapons familiarization.

CNN got exclusive access inside the Marine Security Guard training program at Quantico, where Marines are training as the service tries to almost double the number of its members posted at embassies and consulates worldwide.

A Marine trainee is sprayed with pepper spray as part of the Marine Security Guard training.

The 2013 Defense Authorization Act, which President Barack Obama signed in January, approves the addition of 1,000 Marines to the current force of 1,200.

Plans to increase the size of the force were under way at the time of the September 11, 2012, attack on the U.S. diplomatic compound in Benghazi, Libya, resulting in the death of Ambassador Chris Stevens and three other Americans. Marine security guards were not assigned to the post in Benghazi when it was attacked, and it is unclear whether Marine guards could have helped or even prevented the attack.

Marine security guards are currently posted at embassies and consulates in 137 countries, with a total of 152 compounds being protected. The Corps works directly with the State Department to determine locations Marines will guard.

State Department officials recently announced Marine Corps guard protection for approximately 50 additional embassies.

"We also will create a quick-response unit that will be able to augment embassy security in case there are indications ... in a particular country ... and that ambassador wants reinforcement with more Marines," Col. Michael Robinson, commanding officer of the Marine Corps Embassy Security Group, told CNN's Barbara Starr.

The Security Augmentation Unit will consist of approximately 100 guards that can be dispatched on a moment's notice. They will be on a "quick tether," according to Robinson.

"So instead of requesting through the normal military channels...they would come directly to us, and we would be able to provide this security augmentation unit to help reinforce that particular embassy or consulate," he said.

Marine security guards do not stand guard outside of their assigned embassy. Instead, local guards provided from the host country are designated to protect the outside of the building(s). "We're responsible for mainly internal security within a compound," Robinson said.

Although most of these enlisted trainees are young -- typically 20 or 21 years old -- security guard is not an entry-level position. It is considered a "special duty" assignment, meaning applicants must first work in their trained specialty -- like infantry or mechanic work -- before becoming a security guard.

He or she must be single. Additionally, Marine security guidelines state that guards must have maturity, judgment, moral character and financial stability. An applicant with a lot of debt will not be accepted into the program. In addition, the Marine must have top secret security clearance and submit to a polygraph. Once they graduate, these Marines must do three years of embassy security duty.

On a recent sunny spring afternoon, Robinson watched over a group of Marines in training.

"How you doing? Not too bad, right? Didn't kill you, right?!" he asked a corporal who'd just been pepper-sprayed moments earlier.

"No sir, still alive, sir," the young Marine responded.

"You ready to go out to post? Where do you want to go -- which post?" Robinson asked.

"It doesn't matter sir, as long as I get to serve." he replied.