The historic action by Congress repeals the requirement, known as "don't ask, don't tell," that for the past 17 years has allowed gays and lesbians to serve, but only if they kept quiet about their sexual orientation. Ending that policy has been compared in its social implications to President Harry S. Truman's 1948 executive order that brought racial equality to the military.
After Obama signs the legislation - passed by the Senate on Saturday - into law, the Pentagon must still certify to Congress that the change won't damage combat readiness.
So, for the time being the restrictions will remain on the books, though it's unclear how fully they will be enforced. Some people believe gay discharge cases will be dropped as soon as Obama signs the law. Military leaders, who have been divided on the issue, gave indications that the policy change will be aggressively pursued.
Marine Corps Commandant Gen. James Amos, who had argued against the policy change, said in a statement Sunday the Corps "will step out smartly to faithfully implement this new policy" and that he would "personally lead this effort, thus ensuring the respect and dignity due all Marines."
The issue of gays in the military has been a contentious one for decades. Until 1993, all recruits had to state on a questionnaire whether they were homosexual; if they said "yes," they could not join. More than 13,500 service members were dismissed under the law.
In the 17 years since the "don't ask, don't tell" policy went into effect, views toward the gays in the broader society have evolved. Gay marriage is now legal in five states and the District of Columbia. Opinion surveys say a majority of Americans think it's OK for gays to serve in uniform.
Peter Mansoor, a retired Army colonel who commanded a brigade in Iraq, said he believes the military - from top commanders to foot soldiers - will accept their new orders.
"Pretty much all the heated discussion is over and now it's a matter of the more mundane aspects of implementing the law," Mansoor, a professor of military history at Ohio State University, said in a telephone interview.
Aaron Belkin, director of the Palm Center, a research institute at the University of California, Santa Barbara said only three steps are needed to assure a smooth and quick transition: an executive order suspending all gay discharges, a few weeks to put new regulations in place, immediate certification to Congress that the new law will work. But he said the military may require months of education and training well into 2011.
Defense Secretary Robert Gates, who has supported the change but has stressed a go-slow approach, said "successful implementation (of the new policy) will depend upon strong leadership, a clear message and proactive education throughout the force."
A leading opponent of repealing the 1993 law, Elaine Donnelly, has called the expected certification a "sham" because it will be done by three people who already have stated their support for the change: Obama, Gates and Adm. Mike Mullen, chairman of the Joint Chiefs of Staff.
While gay rights activists say the complications and uncertainties are being overblown, others predict problems.
"The acceptance of open homosexuality and the creation and enforcement of new policies could be far more difficult to implement than repeal advocates ever envisioned," said Richard L. Eubank, a retired Marine and Vietnam combat veteran who leads the 2.1 million-member Veterans of Foreign Wars.
A yearlong Pentagon study on the impact of repealing the 1993 ban said that issues of sexual conduct and fraternization can be dealt with by using existing military rules and regulations, and it found that two-thirds of service members surveyed didn't think changing the law would have much of an effect on military effectiveness. Of those who did predict negative consequences, most were in combat elements such as the infantry.