Women can now serve on US submarines, after a ban was lifted.
The US defence department had announced the move in February and the deadline for any objection from Congress passed at midnight on Wednesday.
Training women for their new duties and the creation of appropriate quarters mean it could be more than a year before women take up their posts.
The cramped conditions had previously precluded women, despite serving alongside men on surface ships.
About 15% of US navy personnel are women.
Defence Secretary Robert Gates had given Congress notice that the ban would end unless it objected and that deadline passed at midnight on Wednesday.
Navy Secretary Ray Mabus said in a statement: "There are extremely capable women in the navy who have the talent and desire to succeed in the submarine force.
"We literally could not run the navy without women today."
The first women submariners are likely to be aboard ballistic and guided-missile submarines as they have more living space and will not require alterations.
However, there is a 15-month submarine officer training course.
Chief of Naval Operations, Adm Gary Roughead, said: "It would be foolish to not take the great talent, the great confidence and intellect of the young women who serve in our navy today and bring that into our submarine force."
The US navy has also announced another key change to service requirements.
As of 31 December, smoking will no longer be allowed on submarines.
There are estimated to be more than 5,000 submariner smokers but they will be expected to fall in line with the general ban on indoor smoking the military imposed in 1993.