US troops are to serve up to 15 months in Iraq and Afghanistan instead of the usual 12-month tours under new rules.
Stretched thin by four years of war, the move is aimed at maintaining troop numbers in the two countries.
The change was announced by US Defence Secretary Robert Gates. Rather than continue to cut home leave intervals.
The longer tours will affect about 100,000 soldiers currently in Iraq and Afghanistan, plus thousands more who deploy later. It does not affect US marines or the national guard or reserve.
"Our forces are stretched; there's no question about that," Mr Gates said.
The extended tours are a price the army must pay to sustain the troop build-up that President Bush ordered in January as part of his revised strategy for stabilising Baghdad and averting a US defeat.
The army has already been given ever-shorter periods of rest and retraining at home between overseas deployments
Mr Gates said it was too early to estimate how long the troop build-up would last, but that his new policy would give the Pentagon the capability to maintain the higher force levels until next April.
Democrats in Congress, and some Republicans, oppose the build-up and are trying to force Mr Bush to change course. In January, the administration indicated the build-up might begin to be reversed by late this year.
AP