The Bush administration was discussing a multipronged approach Monday to using force as it proceeded cautiously and deliberately in preparing the United States military to become fully engaged in what President Bush called "the first war of the 21st century."

Early discussions have included the possible use of air strikes, the deployment of special forces, and a prolonged campaign that could include a full-scale invasion of Afghanistan or other countries harboring terrorists or allowing terrorists to operate freely within their borders.  Usama bin Laden was fingered by the U.S government as the prime suspect in last Tuesday’s attacks and is believed to be hiding out in Afghanistan.

While the military establishment has been tight-lipped about how its operations may be mobilizing domestically and abroad, some details have emerged. 

The aircraft carrier USS Teddy Roosevelt was making its way to join aircraft carrier battle groups already in the Persian Gulf and the Indian Ocean.

The U.S. was also talking to the Pakistani government about using its airstrips if it wants to proceed with F-15 and F-16 fighter jet strikes. Pakistan abuts Afghanistan, making it a strategic location from which the U.S. could launch strikes or amass troops for an invasion.  As of Monday, the Pakistani government had promised the use of its airspace, but not its airstrips, to the Americans.

If unable to use Pakistani airstrips, the U.S. military could launch air strikes from much farther away off an atoll deep in the Indian Ocean, from U.S. bases in the Arabian Gulf or from a NATO airbase at Incirlik, Turkey.

President Bush activated an estimated 35,000 of the nation’s 1.3 million reservists and could call up close to 20,000 more for overseas deployment to the Middle East.

Under Bush's authorization over the weekend, 13,000 Air Force, 10,000 Army, 7,500 Marines, 3,000 Navy and 2,000 Coast Guard reservists had been activated for domestic aid and intelligence, including air defense missions, only.

Bush said he would ultimately call up to 50,000 troops, and many speculate that he may choose to send the remaining 15,000 reservists overseas. By law, the president can call up to 100,000 reservists before asking permission from Congress.

Although Pentagon officials would not discuss the details of their developing plans Monday, they had confirmed that elite paratroopers from the 82nd Airborne Division in Ft. Bragg, North Carolina, and 101st Airborne Division from Ft. Campbell, Kentucky, had been placed on alert.

Experts said it is those two divisions, as well as Special Operations Command (SOCOM) forces like the Navy Seals, that would be sent over first in an invasion, but they also hesitated to speculate on what form the strikes would take.

Jay Farrar, a military analyst for the Center for Strategic and International Studies, said any sustained military strikes would "involve a lot of people in small groups in a lot of places over a long period of time." Farrar cautioned against immediate air strikes that would "only serve the public appetite" for revenge.

"This is going to be a military-slash-law-enforcement operation," offered Farrar. The strikes should be only against those who are part of the "virus" of terrorism, he added. "You rain your terror down on the perpetrators, not on innocent people."

Tom Keaney of the Johns Hopkins University School of Advanced International Studies said the United States would most likely engage in a "building up" of forces in the area rather than a "massive invasion."

"It all depends on the reactions from the various governments – I think that is all developing," he said. While the Bush administration may choose to begin with air attacks and a small insertion of forces, a larger invasion "is more than can be done overnight."

"I think before you plan a big operation for putting troops on the ground … you have to have an understanding of what they are going to do when they get there," he said. "That’s going to take a lot of planning."

The U.S. military has air, land and sea operations all over the globe, including Operation Northern Watch, which is engaged in monitoring Iraq.

Fox News' Steve Centanni and the Associated Press contributed to this report.