There’s a war on, with the Congress debating the terms of the formal resolution of approval for military action against the Islamic State horrors in the Middle East.

But that’s not the only thing on the plate of the nation’s 25th secretary of the Defense Department, and it may not even be the longest war that Ashton B. Carter is involved with.

He will have to fight the Battle of the Budget, and that is where his real qualifications to head the Pentagon come from. A technocrat from academia, Carter was the No. 2 in the Pentagon for two years and oversaw the seemingly eternal spending conflicts among the secretary’s office and the myriad defense contractors and agency heads — and members of Congress protecting their bases or local defense industries.

No one can say today how long the evolving battle against the Islamic State terrorists will take, but the struggle over defense spending is at least as complex. It’s just as difficult to declare victory over an expensive weapons system at home as it is to put down a foreign enemy.

The Defense Department was created in the Truman administration, consolidating leadership that had been split among Navy and War departments from the earliest days of the Republic. So if the nation has had only 25 heads of that department, four of them have served President Barack Obama — an unusually high turnover.

Part of that is because of White House dissatisfaction with the outgoing Chuck Hagel, a former Republican senator who had the distinction of being the only Defense secretary to have served in the enlisted ranks. Obama’s first secretary was Robert Gates, a GOP holdover who served during a time of intense fighting in Afghanistan and Iraq; he was succeeded by Leon Panetta, who retired two years ago.

What Carter shares with all his predecessors is the difficulty of getting a grip on the spending by the vast “military-industrial complex,” President Eisenhower’s term.

We wish him well, as too often defense spending is misdirected; the department needs to spend wisely in a time of budget deficits. The nation’s defense requirements will include replacements for aging vessels in the Navy and bombers like those at Barksdale Air Force Base in Shreveport dating back decades. Yet replacements are ever-more costly, meaning the oversight of Defense contracts is vital as a spending matter; it’s also a political issue if a particular weapon or engine is “sourced” to states and localities protective of the jobs and payrolls.

This can be an arcane issue, but it has real-world consequences, as the inadequate armor for Army vehicles caused death and wounds for our troops in Iraq and Afghanistan. Procurement reform is serious business.

Carter is perhaps among the best qualified ever to tackle the Battle of the Budget. We wish him well.