Defense Secretary Robert Gates said Thursday that the military will reduce its active-duty ground force by as much as 47,000 troops starting in 2015, part of a long list of defense cuts the secretary is pledging, to meet Washington's demand for deficit reduction. 

During a lengthy news conference, Gates projected that the Pentagon has so far reduced its budget over the next five years by $78 billion. He said he's trying to strike a balance between addressing the nation's "dire fiscal situation" and ensuring the country can defend itself and aid allies overseas. 

But with the war in Afghanistan still raging and threats emerging elsewhere in the Middle East, the prospect for steep defense cuts remains a volatile issue. The secretary himself acknowledged Thursday that over the five-year timeline, "conditions around the world can change." The same day Gates announced the budget cuts, the Pentagon confirmed that another 1,400 Marines will soon deploy to Afghanistan to join the surge force that's already there. 

Such developments raise questions about the extent to which the White House and the new Republican majority in the House can hold down -- or at least pay for -- defense costs as they pledge to rein in spending. According to a recent congressional report, each troop in Afghanistan is estimated to cost the federal government close to $700,000 per year. And the amount of money Gates says he's saving over five years is dwarfed by the rising cost of the Afghanistan war -- projected at close to $120 billion this year. 

The unwieldy defense budget is the single largest piece of discretionary spending in the federal budget. All told, it accounts for one-fifth of total spending out of Washington. The Iraq and Afghanistan wars have gradually added more and more to that piece of the budget pie over the past decade, often through off-budget supplemental requests. The passage of the $34 billion supplemental last summer, mostly covering the cost of the Afghanistan troop surge, brought total war spending since 2001 to over $1.1 trillion. 

Given the impact and size of military spending, Republican congressional leaders recently have opened the door to further defense cuts on their watch, without giving too many specifics. 

"Most of my members don't think any department should necessarily be off limits," Senate Republican Leader Mitch McConnell of Kentucky said Thursday, when asked about the defense budget. McConnell said he'd look at what Gates is recommending. 

That was after House Majority Leader Eric Cantor, R-Va., said Tuesday that "everything has got to be on the table" when asked about military spending. 

Cantor spokeswoman Laena Fallon told FoxNews.com in an e-mail that Cantor was "pleased" with Gates' budget announcements Thursday. 

"Our top priority is to ensure that our armed forces at home and abroad are equipped with the tools, training and resources necessary to accomplish their mission and successfully prepare for future challenges. That said, Leader Cantor is encouraged that the secretary is committed to running a fiscally sound Department of Defense and hopes all federal agencies will do the same," Fallon said. 

If all goes according to plan, Gates is looking to reap $78 billion in cuts over the next five years. Much of the savings would come from a controversial downsizing of the military starting in 2015 -- by which point the U.S. hopes to have handed control of Afghanistan security to Kabul. The plan is to reduce the U.S. Army by 27,000 troops and the Marines by between 15,000 and 20,000. 

Gates said Thursday that even if those reductions are made, the military would still have almost 40,000 soldiers more than when he first came on as defense secretary in the final leg of the Bush administration. 

The Pentagon also proposes increasing health care premiums for working-age retired servicemembers to numbers that more closely resemble those in the private insurance industry. 

Meanwhile, the military services have identified $100 billion in waste that could be used for higher priority projects. That means the services will get to keep the savings, Gates said, after cutting big ticket items such as the Marines' amphibious Expeditionary Fighting Vehicle and the Army's SLAMRAAM surface-to-air missile program. 

Also of note, production of the Marine's variant of the F-35 fighter jet will be pushed back two years with the possibility of canceling it altogether if it continues to go over budget. 

"We must come to realize that not every defense program is necessary, not every defense dollar is sacred and well-spent, and that more of nearly everything is simply not sustainable," Gates said.