Defense Secretary Chuck Hagel approved deactivation of the 2nd Infantry Division's 1st Armored Brigade Combat Team effective next summer, officials said. The unit, the so-called "Iron Brigade," has been permanently stationed in South Korea since 1965, staffed by individual soldiers sent to serve a year.

Army Col. Steve Warren, a Pentagon spokesman, said the move was long-planned and did not represent a reduction in U.S. commitment to South Korean security. In fact, he said, similarly sized, fully trained units would be rotated into South Korea for nine-month tours.

Defense officials said the rotation of units that had trained together beforehand, rather than individuals who had to get to know their fellow soldiers upon arrival, could improve unit cohesion and readiness of U.S. forces in South Korea.

"There's not loss in capability," Warren said. "Some would argue that the capability might even be slightly higher because it's a trained unit that arrives there in Korea."

Warren said the first rotational unit would be the 4,600- member 2nd Brigade Combat Team from the 1st Cavalry Division at Fort Hood, Texas. It is due to begin its tour in South Korea in June 2015.

The shift is part of an Army plan originally conceived in 2013 to have some brigades overseas on a rotational basis rather than stationed permanently abroad. A continuous rotational presence would enable different units to gain experience training with allies.

The decision to deactivate the "Iron Brigade" in South Korea is part of the Army's effort to cut the overall size of its force as a result of budget reductions enacted in 2011.

The action will reduce the need for 4,500 military jobs. Soldiers currently with the brigade would be deployed to other units. But Army officials said it would allow the service to shed that number of positions by attrition and other means, moving closer to its planned force size.

The Army currently has about 505,000 active duty soldiers and is in the process of shrinking to 490,000. It is expected to reduce further during the coming year, dropping to between 440,000 and 450,000.

The job cuts come as the Pentagon tries to reduce projected spending by nearly a trillion dollars over a decade. Congress and the president agreed to the cuts in the 2011 Budget Control Act.