King is a free agent after this season and Washington was not going to get a compensation pick for him. So the team felt it was best to trade him now.
"He's a free agent in three weeks. He's not a Type A or Type B free agent," general manager Jim Bowden said. "So it was an opportunity for us to make a deal and get a prospect back. As I told Ray, 'You're now in a pennant race. You're now a game and a half out.'"
King, who was a valuable situational lefty out of the bullpen, went 1-1 with a 4.54 ERA in 55 games this season. The veteran held left-handed hitters to a .161 batting average.
King returns to Milwaukee, where he went 6-8 with 38 holds, one save and a 2.91 ERA in three seasons from 2000-02. If the Brewers make the playoffs, King will not be eligible for the postseason. In order for that to happen, King would have to have been on Milwaukee's roster by Aug. 31. King will report to the team on Wednesday.
During a conference call, King said he expected to be with the Nationals all season, especially after he wasn't traded before the July 31 trade deadline.
"First, it's a shock," said King. "You see the whole trade deadline come and go and you are thinking, 'OK, finish with this team.' You want to finish with respect -- for yourself and the organization. But anytime you get a phone call from the general manager's secretary, it's not good.
"I have a chance to go to Milwaukee. I understand they are a game and a half out, and they are trying to compete and try to get into the playoffs. That team hasn't been there since 1982. ... Sometimes things happened for a reason and sometimes they don't."
Manager Manny Acta credited King for being a positive influence in the clubhouse and solving problems among the relievers.
"Ray was so good over here, just a good guy to have in the clubhouse, really good with the younger players and just did whatever we asked him to do, so that's the sad part of the business," Acta said.
King told Bowden that he would like to play for the Nationals again.
"I would love to come back," King said. "It's a great situation, but you have to deal with the cards you are dealt. If I get to Milwaukee and have good success and they want to try to do something, in baseball terms, you have to do what you have to do."
The trade means that Arnie Munoz will be Washington's lefty specialist, and the club wants to see if he has a future in that role. Munoz went 3-1 with a 2.56 ERA in 54 appearances with Triple-A Columbus, where opponents hit .229 against him.
Bill Ladson is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.