When he was turned away a third time, Rattan - an observant Sikh with a turban and beard - became suspicious.
"I was, like, I don't know what's going on," he said. "I was very introvert at the time. I never felt the need to fight back. But I said I really want to do this, and you guys are sending me out again and again."
The 31-year-old is now US Army Captain Rattan, since July the head dentist at the Fort Drum base in New York.
In what appears to be a quiet shift, the US military since last year has allowed Rattan and two other Sikhs to serve while retaining their turbans and beards, which are required by their faith.
Rattan and another Sikh who received approval last year - Captain Kamaljeet Singh Kalsi, a doctor - said in interviews that their superiors had welcomed them warmly.
Kalsi, 34, said that on his first day of training at Fort Sam Houston in Texas, a first sergeant pulled him out of the crowd and told the soldiers about the Sikh's long ordeal to enlist.
"These were his words: 'The Army is made up of different shades of green, and if you have any objection to him being here, you need to tell me now,'" Kalsi said. "It was great; everybody clapped."
The US Sikh community - estimated at more than half a million - suffered hate crimes after the September 11, 2001 attacks by assailants who falsely associated the religion founded in India with Al-Qaida leader Osama bin Laden.
"I think the only way for that perception to be eliminated is when young Sikhs come up and say: I want to serve in the military," Rattan said.
"For me, I said whatever it takes, I'm going to fight this thing -- I'm going to serve. Maybe if nothing else comes out of it, people will know who Sikhs are," he said.
Sikhs have a historic military culture and have long kept their articles of faith in the militaries of Britain, Canada and India.
Small numbers of Sikhs for years served in the US armed forces without incident. But in the 1980s, the post-Vietnam War military moved to increase conformity and banned displays of religious identity for new recruits.
The Supreme Court in 1986 upheld the military's right to prohibit a Jewish officer from wearing a yarmulke. In response, Congress approved a law requiring the military to approve soldiers' religious apparel if it is "neat and conservative."
Army spokesman George Wright said that it evaluated each Sikh soldier's request based on "unique facts and individual circumstances."
"It is the Army's policy to accommodate religious practices as long as the practice will not have an adverse impact on military necessity," Wright said.
But lawyers for the men believe the US military has developed guidance - a general guideline, but short of an official policy - to accommodate Sikhs.
Most recently, the Army on August 30 accommodated a new recruit, Simran Preet Singh Lamba, after initially denying him. Lawyers closely watched his case as he will undergo rank-and-file training and is not in the medical field.
"I think the Army, and Defense Department more broadly, took nine months to take this decision because it was a big decision," said Amandeep Sidhu, a Sikh American lawyer whose firm McDermott Will & Emery represents the men pro bono.
Sidhu voiced hope that eventually the army would "go that one step further and amend the uniform regulation in a way that would allow Sikhs to serve without having to automatically go through the extraordinary hoops."
Critics have in the past argued that the military needs to ensure conformity and that easing rules could be a slippery slope with precedent for other issues. President Barack Obama's administration is cautiously moving to allow gays to serve openly in the military, a hot-button political issue.
Sikh activists have enlisted the help of the US Congress. Forty-one House members and six senators wrote to Defense Secretary Robert Gates last year to voice concern after Kalsi and Rattan initially heard they could not be accommodated.
"No one should have to choose between his religion and service to our country," said Representative Carolyn Maloney, a Democrat of New York.
Harsimran Kaur, legal director of The Sikh Coalition, a rights advocacy group, hoped that pressure from Congress and closed-door talks with the military would bring an official change in policy.
"A lawsuit is always a means of last resort as it automatically puts both sides in an adversarial posture," she said. "We would very much hope that the Army makes this change on its own, because we want them to understand it's in their best interest as well."
"It shows folks abroad that the United States and the US Army practice what they preach, that we not only tolerate religious diversity and freedom of religious, but we celebrate it," she said.
Captain Kalsi, who recently graduated from training and will soon start serving as an emergency doctor at Fort Bragg in North Carolina, said that religious tolerance "goes to the core of being an American."
And for Sikhs, the military "is in our blood," said Kalsi, whose father, grandfather and great-grandfather served in the Indian or British militaries.
"I want to be able to pass this on," Kalsi said. "I want my son who's now a year-and-a-half old to someday say - yeah, I'm fifth-generation military."