A green-water navy is a maritime force that is capable of operating in its nation's littoral zones and has limited competency to operate in the surrounding marginal seas. It is a relatively new term, and has been created to better distinguish, and add nuance, between two long-standing descriptors: blue-water navy (open oceans) and brown-water navy (inland and shallow littoral waters).
As a non-doctrinal term with no concrete legal or political definition, it can be used in several different ways. It originated with the United States Navy, who use it to refer to the portion of their fleet that specializes in offensive operations in coastal waters. Nowadays such ships rely on stealth or speed to avoid destruction by shore batteries or land-based aircraft.
The US Navy has also used the term to refer to the first phase of the expansion of the Chinese Navy into a full blue-water navy. Subsequently, other authors have applied it to other national navies that can project power locally but cannot sustain operations at range without the help of other countries. Such navies typically have amphibious ships and sometimes small aircraft carriers, which can be escorted by destroyers and frigates with some logistical support from tankers and other auxiliaries.
Definitions
The elements of maritime geography are loosely defined and their meanings have changed throughout history. The US's 2010 Naval Operations Concept defines blue water as "the open ocean", green water as "coastal waters, ports and harbors", and brown water as "navigable rivers and their estuaries". Robert Rubel of the US Naval War College includes bays in his definition of brown water, and in the past US military commentators have extended brown water out to from shore.
During the Cold War, green water denoted those areas of ocean in which naval forces might encounter land-based aircraft. The development of long-range bombers with anti-ship missiles turned most of the oceans to "green" and the term all but disappeared. After the Cold War, US amphibious task forces were sometimes referred to as the green-water navy, in contrast to the blue-water carrier battle groups. This distinction disappeared as increasing threats in coastal waters forced the amphibious ships further offshore, delivering assaults by helicopter and tiltrotor from over the horizon. This prompted the development of ships designed to operate in such waters – the and the littoral combat ships; modeling has suggested that current NATO frigates are vulnerable to swarms of 4-8 small boats in green water. Rubel has proposed redefining green water as those areas of ocean which are too dangerous for high-value units, requiring offensive power to be dispersed into smaller vessels such as submarines that can use stealth and other characteristics to survive. Under his scheme, brown water would be zones in which ocean-going units could not operate at all, including rivers, minefields, straits, and other choke points.
As the preeminent blue-water navy of the early 21st century, the US Navy is able to define maritime geography in terms of offensive action in the home waters of its enemies, without being constrained by logistics. This is not true for most other navies, whose supply chains and air cover typically limit them to power projection within a few hundred kilometers of home territory. A number of countries are working on overcoming these constraints. Other authors have started to apply the term "green-water navy" to any national navy that has ocean-going ships but lacks the logistical support needed for a blue-water navy. It is often not clear what they mean, as the term is used without consistency or precision.