FORT POLK, la. — Shortly after dark, the paratroopers jumped out of C-130s into a Caspian Sea country teeming with mayhem, political unrest and insurgents. Their first mission was to prevent a U.S. consulate from being overrun. Then they were to repel an invasion by a hostile neighboring nation that was after the oil wealth of the fictional country of Atropia.

If all went according to plan, the mission would last no longer than a few weeks.

The training exercise, which kicked off at this U.S. Army base in Louisiana last week, is among the first the Army has designed in an effort to overhaul the country's fighting force as the war in Afghanistan draws to a close.

The withdrawal of U.S. combat forces from Afghanistan by the end of 2014 will conclude a chapter of expensive and unpopular war in that country and Iraq that began more than a decade ago and led to the deaths of more than 6,000 American service members.

The new Army, senior military leaders say, must become more nimble, its officers more savvy, its engagements more nuanced and almost certainly shorter.

The lessons of the Arab Spring weigh on war planners, with threats looming in the Middle East and elsewhere. A high premium is being placed on devising the proper use of Special Forces, drones and cyber capabilities.

"My premise is that the world is going to get more complex, it's going to get more difficult," said Army Chief of Staff Gen. Ray Odierno on Tuesday en route to Fort Polk, where he observed the first phase of the training exercise. "We're going to need leaders who can be very adaptive."

The transition is fraught with challenges. The Pentagon has been ordered to slash its budget by $487 billion over the next decade. As part of that effort, the Army intends to shrink from its 2010 wartime peak of 570,000 active-duty soldiers to 490,000 in 2017.

For the past decade, Fort Polk and other Army training centers have mainly prepared soldiers for the type of challenges they would face in Iraq and Afghanistan.

But it's hard to tell what the next major conflict will look like, so the new training exercises encompass an amalgam of threats, military officials say.