Shrinking the Army. The Pentagon wants to reduce the service's active force to between 440,000 to 450,000 soldiers—making it the smallest force since before World War II, but also roughly consistent with the minimum number that top Army officials say is needed.

Republicans, however, have criticized the request for ignoring, what they view, as an increasingly dangerous world. Taking an early swing at the cuts, Sens. John McCain and Lindsey Graham said in a joint statement that "now is not the time to embrace a defense posture … which left us unprepared to face gathering global threats."

Base closures. Defense officials are already hearing that a request for a round of base closures and realignment in 2017 is dead on arrival at Congress's doorstep.

Base changes—which provoke bipartisan disapproval—face two problems. First, members are loath to be seen cutting jobs in their home state. And while closing bases saves money in the long term, a fiscal-conscious Congress would have to approve spending $1.6 billion through fiscal 2019.

Personnel changes. Congress shot down a recent attempt to reduce some military pensions, but the Pentagon is taking another swing at changes to military personnel costs.

The department wants to scale back its housing allowance to cover 95 percent of estimated rental costs; freeze top brass' pay for a year; and increase "modestly" the health care co-pays and deductibles for retirees and active-duty family members.

Although Tuesday's budget request stresses that the Pentagon covered a smaller percent in the '90s, it moves away from a decade-plus in which personnel spending was largely considered immune to budget cuts.

Retiring the A-10. The Air Force's A-10 aircraft, often used to support troops on the ground, is set to be retired, creating a predictable fight with Republican Sen. Kelly Ayotte of New Hampshire leading the way.

The Pentagon is hoping to use the retirement to save $3.5 billion, but such a plan has been blocked before. As part of an annual defense bill passed in December, the retirement, preparation to retire, or storage of A-10s—except those marked for retirement in April 2013—was forbidden during the 2014 fiscal year.

But the Pentagon could have one advantage. Despite some early backlash, if Congress intends to stick by its base spending cap—as suggested by House Armed Services ranking member Adam Smith—members will eventually have to swallow tough cuts. The ever-important questions of where, and how, remains to be seen.