The US Navy and Congress are in a sort of faceoff over the fleet's cruiser force. To extend their service lives, the Navy is asking to take half its cruisers -- CGs in Navy-speak -- out of service now and gradually bring them back starting in 2019. Congress, fearful that Pentagon budget-cutters will instead decide to cut costs and reduce the force, is insisting the ships be modernized now and kept running.

A level of discomfort -- if not outright distrust -- has been created as the service changed its original 2012 request to decommission seven cruisers under a spending reduction strategy to one where the Navy wants to keep them, but temporarily inactivate 11 its 22 Ticonderoga-class CGs under a modernization plan. Many on the Hill suspect that behind the rhetoric, there lurks a desire to save money by killing the ships.

Meanwhile, production of new DDG 51 Arleigh Burke-class destroyers continues. To some, the DDGs, equipped with more up-to-date versions of the same Aegis combat system that equips the cruisers, seem up to the task of replacing the older CGs. But the Navy insists it needs its cruiser force, and the issue brings up some fundamental questions: What is a cruiser, what's the difference between a cruiser and a destroyer, and what ship will protect the fleet's aircraft carriers in the 2030s?

A US aircraft carrier on deployment is never alone. Like a protective bodyguard, there's always a dedicated warship hovering nearby, rarely beyond the horizon, watching out for any threat and ready to strike if necessary.

The destroyers in the carrier's strike group often will disperse -- sometimes on tasks that take them hundreds of miles away. But a missile cruiser is always riding shotgun, commanded by a senior officer acting as the strike group's air warfare commander -- a critical role in the defense of the carrier.

But the Navy's force of 22 cruisers is aging, and with lifespans of about 35 years, the last of the ships will wear out and leave service by the end of the 2020s -- long before replacement ships are in service to guard the fleet's flattops.

No cruiser replacement is in the works. The Navy had begun development of the CG(X) next-generation cruiser that would have taken over the air defense role, but the program was canceled in 2010 after the projected ships grew too big and too expensive.

It was then hoped that a new Flight III version of the Arleigh Burke destroyers might fill the role. Equipped with a new air missile defense radar, the Flight III would have significantly greater electrical power needs than existing DDGs, and the Navy debated building a larger version of the ships. But in October, the service announced its decision to put the air missile defense radar on standard DDG hulls, and the ships will be poorly suited to embark the extra staff and provide proper command and control facilities for the air warfare commander.

"So the question is, who is going to fill the air defense commander void?" asked Rear Adm. Thomas Rowden, the Navy's director of surface warfare.

Under the proposed modernization plan, Rowden said, the reduction to 11 active cruisers means a destroyer would fill the secondary role. But the DDGs are somewhat limited in taking on that mission.

"We have done air defense with missile destroyers before," Rowden said. "And clearly, we could take our destroyers and to a certain extent increase the level of expertise on those ships by putting a captain in charge. But the density of the ship, the ability to add staff to the ship, the reduced command, control and communications equipment on our destroyers really makes them not as optimal an air defense commander ship as our cruisers."

Rowden ticked off other factors. Destroyers, he pointed out, have only one radar transmitter, and all four radar arrays are on a single deckhouse. The cruisers split the radar arrangement, with two arrays and a transmitter in each of two deckhouses, providing redundancy in case of battle damage. And cruisers have more missile cells than destroyers, with four target illuminators rather than three.

Cruiser communication suites -- "radio circuits, satellite communications circuits" -- are greater than a destroyer's, Rowden noted. Extra space for the air defense commander's staff also is available on a cruiser -- space in the combat information center, with 20 consoles compared with a destroyer's 16, and space in accommodations areas.

Operationally, destroyers are called upon to defend other fleet units, including amphibious and logistics ships -- not a role for cruisers, Rowden said.

"It does not make sense to me to take a cruiser and all of the capability, capacity and expertise on that ship and use it to defend logistics, the sea lanes, the communications to bring support materials as they operate forward. But I see that as a significant role for the destroyers," Rowden said.

Capt. David McFarland, Rowden's deputy in the Surface Warfare Division, is an experienced cruiser and destroyer commander.

"You can use a DDG as a shotgun, but only in a tactical sense, not a command-and-control sense," he said. "As a destroyer captain, I've been shotgun for a carrier, and I did it well, it's just maneuvering. But I was also the area air defense commander when a cruiser wasn't around and that is extremely difficult."

Work on a CGR replacement cruiser isn't expected to start for at least a decade, Navy leaders point out, with funds increasingly committed to design and build nuclear-powered ballistic missile submarines to replace the Ohio class. That means a CGR isn't likely to be fielded until the mid-2030s, at the earliest.

The phased modernization plan, Rowden said, would essentially stop the lifecycle clock of inactivated ships. Refurbished and modernized cruisers would be returned to active duty in time to replace older ships as they reach the end of their service lives.

If nothing is done, the Navy had planned for the final CGs to leave the fleet by 2028. Under the phased plan, the 11 ships returned to service would leave active duty between 2035 and 2045, providing a significant window to develop and deploy a new design.

But buying in to the plan remains difficult on Capitol Hill, where opposition is widespread. The change in the rationale to inactivate the ships, along with the Navy's tardiness this spring in presenting its phased modernization plan to Congress, has made it tough for some to swallow.

"They wanted to get rid of them, then overnight they came up with this plan," said one staffer, who noted that the Navy briefed the Hill on details of the plan only just before the 2015 defense bill markups began, making it difficult or impossible to incorporate its implications. And the latest version of the 30-year shipbuilding plan, sent to Congress July 1, provides few details of the proposed plan.

"The track record on a variety of issues is not great," complained the staffer of the Navy.

Compounding the communication problem, the Navy is striving to indicate the cruisers would not be officially decommissioned , only put into some sort of caretaker status pending their modernization and reactivation. The search for a proper term has been difficult -- there is little precedent for inactivating ships yet continuing to count them on the active roster.

"You can't guarantee that Navy leaders won't look at that [inactivated] ship downstream and think 'I don't want to pay to bring that ship back in service,' " said another congressional staffer. "That ship will look old to them by then." ?