Washington — Sen. Mark Warner is all for defense modernization. But just don’t touch those aircraft carriers, six of which are based in Norfolk.

The Virginia Democrat had said a year ago year that rather than investing in 20th-century military technology, he wanted to discuss “a reallocation of some of those resources” to deal with the 21st-century challenge of cyber threats. But when the Navy this week proposed to retire the carrier Harry S. Truman, to save money for modernization, Warner urged it to “reassess the decision.”

This battle between past and future is the hidden drama within the gargantuan $750 billion fiscal 2020 defense budget proposal. Nearly everyone favors high-tech weapons to combat great-power adversaries in the new millennium, in principle. But meanwhile, the military-industrial-congressional complex, as the late Sen. John McCain termed it, keeps pumping vast sums to sustain legacy weapons systems.

The 2020 budget, shaped by acting Secretary of Defense Patrick Shanahan, does propose some important changes. Spending for space programs will increase about 15 percent, and cyber programs will rise 10 percent. Yes, the Navy wants to add two new carriers while dropping the Truman, but it also proposes to build two big drone warships and some unmanned subs. The Army plans to cut or reduce 93 outmoded programs for vehicles, weapons and helicopters to make room for modernization.

“Overall, I think it’s moving in the right direction,” says Christian Brose, former staff director of the Senate Armed Services Committee. “The emphasis on modernization is clear and welcome.”

But Brose offers some “caveats” about how the process will play out. The budget proposes $104 billion for research, development, testing and evaluation, the biggest R&D request since the 1940s. But Brose says some of this will be “old wine in new bottles,” like spending for the wildly expensive F-35 fighter program, rather than disruptive new technology.

Some high-tech spending involves classic Pentagon log-rolling among the military services. Take, for example, “hypersonic” weapons, the bright shiny objects (literally and figuratively) that will be streaking across Pentagon skies. The 2020 budget proposes to increase spending on hypersonic weapons to $2.6 billion from $2.4 billion (good), but gives the Army, Navy and Air Force their own hypersonic systems (crazy). The argument is that America will want air, land and sea versions, but why?

When there’s new technology around, every service wants a piece of the action. Pentagon old-timers recall the battle in the 1950s between Army and Air Force generals about whether missiles were munitions or air weapons.

A deeper worry is whether some of the modernization spending in the 2020 budget will be thrown overboard to preserve legacy systems when Congress starts cutting the $750 billion Trump administration request, as Democrats will demand. The final figure will probably be many tens of billions less, and powerful members of Congress will fight to preserve the aircraft carrier and F-35 procurements that protect jobs in their states and districts.

“Often, it’s the future that ends up without a chair when the music stops,” says Brose. He now works for a startup company called Anduril Industries that wants to be a disrupter. It has built a system that uses artificial intelligence to fuse sensors and drones to solve defense problems, such as perimeter security at military bases or along the border, more effectively and cheaply than conventional systems. The Pentagon seems interested, but Congress will have the final say.

Military leaders are working to adapt old platforms to meet new challenges, and that’s both good and bad. Adm. John Richardson, the chief of Naval Operations and a consistent advocate of innovation, says the Navy has made aircraft carriers stealthier, using electronic warfare and other measures, and by the end of this year will start arming carriers with lasers that can shoot down attacking missiles and planes.

“The aircraft carrier is less vulnerable than it’s been since World War II,” Richardson told me in an interview last week. He argues that in combating Chinese military power in Asia “the aircraft carrier is the most sustainable airfield in the theater.”

Certainly, America needs aircraft carriers. But does it need 10 of them, as the 2020 budget contemplates (or 11, if Congress preserves the Harry S. Truman, as seems likely)? We need F-35s, too, but is the massive buy in the 2020 budget needed?

The Pentagon can’t have everything it wants, and the danger is that when it’s time to start cutting, the military-industrial-congressional complex will jettison the new weapons we need to keep the old ones we don’t.

— David Ignatius is a columnist with Washington Post Writers Group.