The United States' aircraft carriers have always been an almost untouchable deterrent, steel behemoths capable of projecting the full weight of the military wherever they deploy.

Yet while many militaries could never hope to match the carrier fleet in size and strength, countries such as China, Iran and Russia have spent recent years adjusting their forces and fielding equipment designed to counter one of the United States' greatest military strengths.

A report published Monday by the Center for a New American Security, a D.C.-based think tank that focuses on national security, claims that the Navy's carrier operations are at an inflection point. Faced with growing threats abroad, the United States can either "operate its carriers at ever-increasing ranges … or assume high levels of risk in both blood and treasure."

US Secretary of Defense Ash Carter (R) speaks with US Navy Cmdr. Robert C. Francis Jr., as Carter and Malaysian Defense Minister Hishammuddin Hussein (Not Pictured) visited the USS Theodore Roosevelt aircraft carrier in the South China Sea, in this handout photograph taken and released on November 5, 2015. REUTERS/Senior Master Sgt. Adrian Cadiz/Department of Defense

The report, titled "Red Alert: The Growing Threat to to Aircraft Carriers," centers around China's burgeoning military posture in the Pacific and on a term that is starting to appear with an ever-increasing urgency in defense circles: anti-access/area denial, or A2/AD. The term A2/AD centers around a concept that has long existed in warfare: denying the enemy an ability to move around the battlefield.

Currently A2/AD strategy is as similar as it was when moats were dug around castles, except today's moats are an integrated system of surface-to-air missiles, anti-ship cruise missiles, submarines, surface ships and aircraft all designed to push enemy forces as far away as possible from strategically important areas.

The report focuses on China's capabilities because of its "emphasis on long-range anti-ship missile procurement." This, coupled with its growing tech base, qualifies China as the "pacing threat" to the military. China, however, is not the sole architect of an A2/AD strategy designed to deter operations.

Russian President Vladimir Putin delivers a speech during celebrations for Navy Day as it rains in Baltiysk Thomson Reuters

In the Baltics, Russia's naval base in Kaliningrad is known to house a sophisticated air defense network and anti-ship missiles. NATO commanders also have warned of Russian A2/AD buildup around Syria, as Russia has moved advanced surface-to-air missiles into its airbase there as well as a flotilla of ships with robust anti-air capabilities.

As other countries focus on creating sophisticated A2/AD bubbles by using new technology such as drones, advanced missiles and newer aircraft, the United States -- by operating as it always has -- is putting itself more at risk. According to the report, this is particularly relevant as carrier groups have reduced their long-range strike ability in lieu of being able to fly more air missions but at shorter ranges.

"Operating the carrier in the face of increasingly lethal and precise munitions will thus require the United States to expose a multi-billion dollar asset to high levels of risk in the event of a conflict," the report says. "An adversary with A2/AD capabilities would likely launch a saturation attack against the carrier from a variety of platforms and directions. Such an attack would be difficult — if not impossible — to defend against."

Military vehicles carrying shore-to-ship missiles drive past the Tiananmen Gate during a military parade to mark the 70th anniversary of the end of World War Two, in Beijing Thomson Reuters

Last week, China's A2/AD strategy made international news after satellite imagery showed the deployment of HQ-9 surface-to-air missiles on Woody Island, a disputed atoll in the South China Sea. Though small, the island is claimed by both Taiwan and Vietnam. The CNAS report classifies the HQ-9 as a short-range A2/AD threat but indicates that the movement of such systems into disputed territory in the South China Sea, if properly reinforced, is a potentially long-term problem for naval operations.

Medium and long-range threats discussed in the report include land-based Chinese bombers and anti-ship ballistic missiles such as the DF-21D and DF-26. The two missiles "represent a significant threat to the carrier," with an estimated range of 810 and 1,620 nautical miles, respectively. According to the report, if the DF-26 is as operational and as accurate as the Chinese say it is, the missile would be able to hit the territory of Guam.

While the report discusses possible countermeasures for a sophisticated A2/AD network, including the Navy's future railgun project, the United States probably would employ a variety of systems and strategies, including hacking, to defeat the enemy threat.

However, long-term strategies suggested in the report include putting combat power into systems such as submarines and long-range carrier-based drones. Submarines could evade A2/AD by remaining undetected, while carrier based drones -- with their increased range -- would give carriers much-needed standoff from potential A2/AD threats.

The United States "must re-examine the relevance of the carrier and its air wing and explore innovative options for future operations and force structure," the report concludes. "If the United States is to maintain its military superiority well into the future, it cannot afford to do otherwise.