Scuffles among eager ex-combatants standing in surging lines outside the army's recruitment centre in Monrovia have broken out daily over the past month.
Peacekeepers swung clubs to restore order.
"When I joined the rebels I did it for my country," said 20-year-old Tanu Soui, who slept on a gritty sidewalk outside the recruiting centre one night to keep his place in line.
"Now I want to serve the new Liberia."
An army placement is invaluable for Liberians with 3 million people without jobs across the country, making up 80% unemployment.
Rumours have swirled about salaries beginning at $US90 ($A120) per month, though no pay scale has officially been announced.
Reconstituting the army and building a professional police force are crucial first steps if Liberia is to escape the cycle of coups and civil war.
"All too often in Liberia's recent past, the police and army served the country's ruling cliques, not its people. This all but guaranteed civil war," said Doug Coffman, a UN spokesman in Liberia.
The new army will number just 2,000 soldiers, while around 100,000 rebels and former government troops were demobilised under a UN-backed disarmament campaign after the war's end in 2003.
Top UN officials have also said the army should be small because its main purpose should be protecting the country's borders from outside aggressors.
But in a country awash with tens of thousands of unemployed ex-fighters and a history of civil war, the main threat may come from within.
Initially planned at 4,000 strong, the new army's size was reduced due to what the United Nations called the government's "acute budgetary constraints".
While Liberia is rich in diamonds, the economy has been devastated by wars that began in 1989, leaving the nation dependent on aid.
Ellen Johnson Sirleaf, Liberia's first postwar president, recently said she expects the 15,000 UN peacekeepers to stay for only two or three more years.
In neighbouring war-battered Sierra Leone, peacekeepers wrapped up a five-year mission last month, leaving behind an army 13,000 strong.
To train Liberia's troops, the US State Department hired DynCorp
International, a Texas-based US defence contractor that has worked with armies in Iraq and Afghanistan.
One day after President Sirleaf took office January 16, DynCorp began screening recruits between the ages of 18 to 45.
With retired American drill instructors barking commands, DynCorp plans to put the recruits through 15 weeks of basic training they hope will make the troops forget that they once fought each other.
"In all militaries, people come from different backgrounds and Liberia is no different," said Renee Hubka, a DynCorp spokeswoman.
"The training is meant to break you down and build you up into a cohesive group."
In addition, the UN is training a 3,500-strong police force at a cost of $US8.5 million ($A11.23 million). So far, 1,800 new police are on the streets, though they aren't allowed to carry guns yet.
After passing an aptitude test and then a fitness course, potential army recruits undergo drug and HIV screening. If they pass, they'll have their names and pictures published throughout the country, giving Liberians one year to pick out any that have committed serious human rights abuses.
The old army had a weak command structure, with soldiers as young as 10 taking orders from boys just a few years older. Many officers had inflated titles - some declared themselves generals - a potential source of conflict in the new force.
Potential recruits say they are eager to put their differences aside and help cement peace.