The wars in Iraq and Afghanistan have cost taxpayers about $1.3 trillion in defense appropriations over the past decade, but that amount represents barely more than half of the growth in the Pentagon’s spending during that period.

A new analysis from Todd Harrison, over at the Center for Strategic and Budgetary Assessments, says increases in military pay and benefits account for 19 percent of the growth; a bump in spending on the modernization and replacement of weapons systems accounts for 16 percent; and a rise in the costs of peacetime operations (the daily affairs and training of the armed forces) makes up another 10 percent.

It has been, as Harrison calls it, “hollow growth”:

The base budget now supports a force with essentially the same size, force structure, and capabilities as in FY 2001 but at a 35 percent higher cost. The Department is spending more but not getting more.

So even as the cost of the wars is expected to subside — the Obama administration’s war funding request for fiscal 2012 is 27 percent less than the amount authorized for fiscal 2011 — the need to find major cuts will persist, especially if the Pentagon is to meet the president’s request to slash $400 billion over the the next 12 years, or perhaps even more.

Defense officials have suggested that much of the savings can be found without cutting into major weapons programs or drastically altering force structure. But congressional pressure to target certain programs is already building, and Pentagon officials have acknowledged there are hard choices on the way.

There is, meanwhile, always the chance that the administration has low-balled its requests for war funding, thereby exacerbating the need to find cuts elsewhere later on. As Harrison notes, a decision by Iraqi officials and their American counterparts to keep U.S. military personnel in Iraq beyond the Dec. 31 withdrawal date would significantly increase costs for fiscal 2012.