"The Marine Corps fills a unique lane in the capability range of America's armed forces," then Commandant James F. Amos wrote in a 2011 memo to US Defence Secretary Leon Edward Panetta.
The unique capacity of the US Marine Corps (USMC) to operate by sea, land and air, and move seamlessly between these three domains, he said, enables the US to respond to unexpected crises at a moment's notice - and for less than 8% of the total DoD budget. The key, Amos explained, is readiness: the USMC was, after all specifically mandated by Congress during the Korean War to be "the most ready when the nation is least ready."
However, it seems that central tenet may be under threat. In March 2016, USMC Assistant Commandant General John Paxton told the Senate Armed Services Subcommittee that the Corps might not be adequately prepared to respond instantly in the face of a sudden, urgent demand.
Two factors in particular are putting the Marines under growing pressure - the ongoing need to deploy significant numbers across the globe, despite a sizeable drawdown in personnel, and the recent Pentagon cutbacks which have impacted on important maintenance and equipment upgrades. Nowhere are these problems more apparent than in marine aviation.
Aviation at breaking point
The Marine Aviation Plan 2016 published in January highlighted the USMC's struggle to keep its planes in the air and its pilots trained, with around 80% of aviation units lacking the number of ready aircraft needed for training and emergency response.
The aircraft in the USMC's tactical fighter and attack squadrons (TACAIR) currently have an average age of more than 22 years - more than twice the average age of the navy's equivalent. Delays in the modernisation programme coupled with increased wear on its ageing aircraft means that the Corps's TACAIR fleet of F/A-18 A-D Hornets and AV-8B Harriers routinely suffers shortages in availability.
This problem has been further exacerbated by 2013's enforced leave for highly-skilled civilian specialists and led to the aircraft requirement to be temporarily dropped from twelve to ten for F-18 squadrons, from 16 to 14 for Harriers, and 16 to twelve for CH-53Es. The goal is to allow home station squadrons greater training opportunities, but as General Paxton told the Subcommittee, it means that the Corps is essentially increasing risk in one area to mitigate risk in another.
The tempo of USMC actions has implications for personnel, too. For those involved with high-demand units, such as the tilt-rotor MV-22 Ospreys and KC-130J aerial refuelling aircraft, it means a deployment-to-dwell (D2D) ratio of 1:2 - two months at home for each month on deployment. The Corp's optimal D2D stands at just 1:3.
Not a lot of slack
Jesse Sloman, research assistant at the Center for Strategic and Budgetary Assessments (CSBA), fears that marine aviation is at risk of slowly breaking under the load. "The Marine Corps is most likely to experience critical gaps in the future in aviation readiness and modernization," he warns. "Two of the Corps' most important new platforms, the F-35 and the CH-53K, have suffered from cost overruns and delays sufficient to force the Corps to fly some of its legacy platforms far longer than intended."
Sloman says that if the timeframes for initial operating capability of the CH-53K or the full operating capability of the F-35B slide further - both of which he believes are highly possible events given the troubled history of the two programmes - marine aviation readiness is likely to suffer even more. It would mean old aircraft are forced to remain in service longer, and require funds to be diverted from long-term upgrade and modernisation programmes to pay for the short-term fixes necessary to keep the legacy platforms in the air.
"In short," he says," there is not a lot of slack in the force."
In November 2015, Sloman co-authored a report with CSBA senior fellow Bryan Clark which, among other things, highlighted the difficulties brought about by the USMC's high operational tempo, and the current mismatch between the supply of naval forces and the demands placed upon them. The Corps' numbers and resources are simply declining faster than its operational commitments: there are now around 5% fewer active Marines compared with two years ago, but only 2% fewer deployed overseas.
In their report, titled 'Deploying beyond their means: America's navy and Marine Corps at a tipping point', Clark and Sloman set out the stark but simple choice between maintaining forward presence at its current level and risking breaking the force, or reducing it and restoring readiness through training, maintenance and optimum D2D.
As it stands, Sloman says, the Marine Corps is essentially being forced to accept an informal tiered readiness model in order to sustain its deployed forces during a period of reduced budgets. Consequently, if an unexpected crisis arises that requires a large-scale, immediate response, the Marines may have trouble fielding forces with the very high standards of proficiency that the service has maintained, across the board, in the past.
He sees this as a particularly worrying development because historically, the US has often been able to rely on high levels of training and state-of-the-art equipment to offset any deficits in overall manpower.
"Reducing readiness in garrison, and undertaking cost-saving options like training units for just a portion of their overall mission set, contributes to the erosion of the qualitative edge that has allowed the US military to be so successful in combat and may increase the risk to Marines deploying to a crisis," Sloman warns.
Preparing for the future
That may become even more relevant as the nature of potential deployments changes. In his evidence to the Senate Subcommittee, General Paxton spoke about the USMC's need to prepare for "the distributed and lethal battlefields of 2025" and advised that the force needed to fight in these was unlikely to be a mirror of today's Corps.
It would require balancing future investment and commitment to experimentation against the demands for current readiness and call for significant, and yet unknown, adjustments in manpower, training and equipment, possibly going some way beyond existing capacities, he said.
Sloman shares this view. He expects future battlefields to feature opponents armed with "anti-access/area-denial" weapons, most notably land-based anti-ship missiles capable of over-the-horizon targeting. These missiles pose a tremendous challenge to the current US method of conducting amphibious operations, which requires large capital ships to operate from a position of extreme vulnerability close to an opponent's coastline. It means that the future Marine Corps will have to incorporate new concepts and systems for bringing personnel and equipment ashore, while allowing landing ships to remain safe far out at sea.
To address these future challenges, Sloman says, the Corps is already experimenting with some innovative ideas, such as the use of landing craft air cushion (LCAC) hovercraft to ferry assault vehicles close to an adversary's coastline, but he thinks this may not be enough. According to Sloman, the modernisation programmes currently in place for surface connectors are very conservative, and will lead to platforms with only marginal capability improvements over the LCAC and the existing assault amphibious vehicle and landing craft utility.
"The navy and Marine Corps may have to think more radically about vehicle and ship designs if they are to create a ship-to-shore architecture capable of operating on the lethal battlefields of the future," Sloman says.
One thing is clear, though: long before then, the stark choice that Clark and Sloman set out in their report will have to be addressed. Given the pace and worldwide scope of the ongoing operational demands placed on the USMC, doing nothing is simply not an option.