The 93-4 vote in the Republican-led Senate - following a similar action by the House - reflected the anxieties lawmakers have been hearing from families of service personnel whose tours in Iraq keep getting extended and whose return to civilian life is repeatedly postponed.
Sen. John McCain, R-Ariz., said the lack of troops at the end of major combat in Iraq cost the military an opportunity to stop the violence that continues today.
"We didn't have enough people on the ground, and now we are paying a very, very heavy price for that incredible mistake on the part of the civilian leadership in the Pentagon of the United States of America," he said.
The increase was approved as an amendment to a $447 billion defense authorization bill. Under the proposal by Sen. Jack Reed, D-R.I., the size of the Army would increase by about 4 percent, to 502,400. Congressional aides estimated the cost at $1.7 billion.
Armed Services Committee Chairman John Warner, R-Va., supported the amendment after it was modified to specify that the money would come from a $25 billion fund within the bill for Iraq and Afghanistan operations or from a future emergency spending bill.
"The Army needs this active duty strength. We are in agreement, I think, on this point," he said.
Army leaders oppose a permanent increase in troop strength, saying they can do the job with the current force once it is organized more efficiently.
The Army also had a higher cost estimate for the additional troops. Army Chief of Staff Gen. Peter J. Schoomaker told reporters this week it costs the Army $3.6 billion a year to keep 10,000 soldiers trained and equipped, and the Army would struggle if given thousands of extra troops without the money to pay for them.
"Congress can only fund us one year at a time," Schoomaker said Tuesday. "They can encumber us forever. We are very reluctant to be encumbered by more than is necessary."
Earlier this year, Defense Secretary Donald Rumsfeld approved temporarily increasing in the Army's strength by 30,000 more than its congressionally authorized size of 482,400. Army officials have said that was necessary only so the Army could fight in Iraq and Afghanistan and reorganize at the same time.
As originally drafted, the Senate bill would have given the Army the flexibility to add 30,000 troops, but wouldn't have required it to do so.
The House version of the defense bill, approved last month, would add 30,000 Army soldiers and 9,000 Marines over three years.
The White House criticized the House provisions. "A mandatory increase would lack flexibility and could leave troop levels higher than needed, especially after several (Defense Department) initiatives to reduce demand on the force have had time to work," it said in a statement.
The House and Senate language will have to be reconciled after the Senate approves its final version of the defense bill.
By Ken Guggenheim