A policy banning open homosexuality in the US military has been repealed after nearly two decades.
The dropping of “don’t ask, don’t tell” means service members can now reveal they are gay without fear of investigation or discharge.
“Repeal Day” parties have been organised across the country to mark the victory for gay rights.
The US Congress voted last year to repeal the law, which was introduced in 1993 under the Clinton administration.
Under the policy, gay people were permitted to serve as long as they did not openly acknowledge their sexual orientation, while commanders were not allowed to ask.
Displays of affection
Pentagon press secretary George Little told reporters on Monday that the Department of Defense was fully prepared for repeal, and that 97% of military personnel had received training on the new law.
The armed forces have been accepting applications from openly gay recruits for a number of weeks and will begin processing them now that the new law has taken effect.
The military has also published a revised set of regulations, without references to any ban against homosexual service members.
Pending investigations, discharges and other administrative proceedings have now been dropped under the new law.
Those who have been discharged under the don’t ask, don’t tell rule are entitled to re-enlist.
But existing standards of personal conduct, such as those pertaining to public displays of affection, will not change.
Nor will there be any change to the eligibility standards for military benefits.
Under current rules, service members are allowed to choose a partner as care-giver under the Wounded Warrior programme, or as a life insurance beneficiary.
“Our nation will finally close the door on a fundamental unfairness for gays and lesbians, and indeed affirm equality for all Americans,” said House Minority Leader Nancy Pelosi, a California Democrat and leading advocate of the repeal.
Some in Congress remain opposed to repeal, arguing it could undermine efficiency and discipline in the military.
After the Don’t Ask Don’t Tell Repeal Act was passed by Congress last December, President Barack Obama, Defence Secretary Leon Panetta and Joint Chiefs of Staff chairman Admiral Mike Mullen certified that allowing openly gay service members would not undermine the military.