Gen George Casey said the world remained "dangerous and unpredictable", and the Pentagon must plan for extended US combat and stability operations in both Iraq and Afghanistan that could deploy 50,000 US military personnel for a decade.
"Global trends are pushing in the wrong direction," Gen Casey said. "They fundamentally will change how the army works."
His planning envisioned combat troops in Iraq and Afghanistan for a decade as part of a sustained American commitment to fighting extremism and terrorism in the Middle East.
Gen Casey's calculations about force levels are related to his attempt to ease the brutal deployment calendar that he said would "bring the Army to its knees". His goal was, he explained, to move rotations by 2011 to one year in the battlefield and two years out for regular army troops, and one year in the battlefield and three years out for reserves. He called the current one-year-in-one-year-out cycle "unsustainable".
Emphasising he was not a policy maker, he was adamant he did not intend to contradict Obama administration policy, which is to bring US combat forces home from Iraq in 2010. The US and Iraq have agreed that all American forces would leave by 2012.
Although several senior US officials have suggested Iraq could request an extension, the legal agreement the two countries signed last year would have to be amended for any significant presence to remain.
The US currently has about 139,000 troops in Iraq and 52,000 in Afghanistan, with a further 16,000 to arrive by the end of this year.
He said his could foresee ten combat brigades plus command and support forces committed to the two wars. Brigades tend to number three to five thousand.
He also said the US had to be careful about what assets get deployed to Afghanistan. "Anything you put in there would be in there for a decade," he said.
The general's duties include main responsibility for assembling the manpower and determining assignments. He insisted the army's size of 1.1 million was sufficient even to handle the extended Mideast conflicts.