New generation warfare
Putin has stated that "the collapse of the Soviet Union was the greatest geopolitical catastrophe of the century." Ever since, Putin has labored to reestablish Russia as a world power and dominance in Eastern Europe, and pushed to restructure the armed forces, and introduced new tactics in the 2000 Second Chechen War.
The Russian Army General Staff redesigned the Russian Army for decentralized operations including the possibility of unconventional operations, conventional weapons, and tactical nuclear environments, in order to face different kinds of threats from the high-tech West, massed armies in the east, and unconventional threats from the south. This was used in 2008 in the Georgian conflict, and practiced in field simulations against NATO, through 2013.

It was also in 2013 that publications by Russian military writers about "new generation warfare" became available, and Chief of the General Staff General Valery Gerasimov called for "rethinking on the forms and methods of warfare." Many analysts consider the Gerasimov doctrine the best description of Russia's modern strategy of total warfare, which uses politics as well as war, and attacks on multiple fronts at once including hackers, business, media, disinformation, as well as military means. The internet has made available the possibility of new tools and approaches to destabilize or influence foreign actors, and the Gerasimov doctrine describes a framework for using them, and says that they are not just adjuncts of military force, but are in fact, the primary or preferred means to victory. The strategy is to foment chaos within an enemy state, and develop permanent unrest and conflict there. West Point analyst John Chambers uses the terms Gerasimov doctrine and new generation warfare synonymously.  Chambers views the Gerasimov/new generation warfare doctrine as having been formulated specifically to exploit the weaknesses of large enemy bureaucracies such as the United States.

The Gerasimov doctrine is a foreign policy doctrine named after a February 2013 publication of an article by Valery Gerasimov, Russian chief of the General Staff, which laid out a new theory of modern warfare combining old Soviet tactics and strategic military thinking about total war which was based more on societal aspects than traditional military ones. Gerasimov wrote:The very 'rules of war' have changed. The role of nonmilitary means of achieving political and strategic goals has grown, and, in many cases, they have exceeded the power of force of weapons in their effectiveness. … All this is supplemented by military means of a concealed character.
The prime example of the use of the Gerasimov doctrine in the field, is the 2014 unrest in Ukraine, where Russia supported extremists on both sides to exacerbate tensions and create unrest, and then used the unrest as a pretext to take over Crimea.

Key aspects: Previous military strategies focused on traditional military concerns such as logistics, strength and location of enemy forces, whereas new generation warfare identifies the battleground as the mind of the civilian public. One analyst wrote:
Thus, the Russian view of modern warfare is based on the idea that the main battlespace is the mind and, as a result, new-generation wars are to be dominated by information and psychological warfare, in order to achieve superiority in troops and weapons control, morally and psychologically depressing the enemy’s armed forces personnel and civil population."
Characterizing this emphasis on the mind, Berzins describes the theory as prioritizing "influence over destruction; inner decay over annihilation; and culture over weapons or technology", and is a "total war battlespace" that includes "political, economic, informational, technological, and ecological" aspects. The goal is to implement the theory via eight phases, starting long before there is any military conflict. "Phase Zero" is about preparing the terrain by implementing psychological, informational, and other measures that prevent conflict.
Chambers views New generation warfare as designed to use grey-zone hybrid threats, and "is focused on using non-military means of achieving political and strategic goals as well as military means 'of concealed character' to include the open use of forces 'often under the guise of peacekeeping and crisis regulation' to accomplish objectives. These tactics and techniques are particularly effective against the United States in the gray  zone due to laws and regulations affecting our ability to act in Phase 0."

Strategic communication: In New generation warfare, communications and the media are used to uphold a narrative aimed both at the international community as well as at a domestic audience, using the language of law to evade the usual deterrence and whip up support at home for operations they characterize as legal uses of force to support "protection and self-defense of Russian nationals" living in Donbas and Crimea, and an invitation for intervention by local leaders there who have been the object of years of softening up by the Russians in the preparatory phases. On the surface, Russia appears or claims to be acting in support of self-defense and sovereignty, as supported by Article 51 of the UN Charter, and supported by acts of the Russian parliament. New generation warfare seeks to take the acts themselves out of the realm of military action, such that it's unclear if an armed attack has, in fact, occurred, confusing their enemy's ability to define the response as a military attack and respond to it as one.

Elements: Defense and national security expert on Ukraine Phillip Karber identified five elements of New generation warfare:

political subversion –  intelligence agents; agit-prop operations using mass media to exploit internal differences; corruption, bribery and intimidation of local officials; kidnapping, assassination and terrorism; creating local opposition cells from discontented elements
proxy sanctuary –  seizing local government centers of power, airports and military depots; arming and training insurgents; creating checkpoints; destroying ingress transportation infrastructure; cyberattacks; phony referendums; establishment of a "People’s Republic" under Russian domination.
intervention –  deploying Russian forces to the border with large-scale exercises involving ground, naval, and air force troops; clandestine introduction of heavy weapons to insurgents; creation of training and logistics camps adjacent to the border; integrating proxy troops into Russian equipped and led formations.
coercive deterrence –  secret strategic force alerts; deployment of tactical nuclear delivery systems; theater and intercontinental maneuvers; aggressive air patrolling of adjacent areas to limit their freedom of movement.
negotiated manipulation –  use and abuse of Western negotiated ceasefires to regroup and rearm their proxies; using violations as a tactic, while inhibiting other states from helping due to fear of escalation; split the Western alliance by using oil or economic incentives; selective and repetitive phone negotiations that go nowhere.
Chambers compared New generation warfare planning processes with American processes, and identified an eight-phases construct on the Russian side, and six phases on the American side. They don't line up well, and the first four phases of New generation warfare all fall within Phase 0 of US planning, the latter requiring a lot of interagency planning and communication, slowing it down, while the Russian process is designed to be nimble and work within the US phases. As Chambers puts it, "The fundamental problem with countering operations in the gray zone takes place when the United States cannot clearly identify gray-zone actors and actions (ambiguity) nor develop a clear common operating picture of actions taking place. In this case, the United States will be constrained by a bureaucracy that will likely not authorize operations until there is a clear threat. At this point, it is too late and the adversary has already used gray-zone operations to shape the battlefield and achieve its strategic objectives."

In practice: The New generation warfare theory became used in practice in Crimea and Donbas in 2014, in Russia's proxy war with Ukraine. Karber stated that "As practiced in Ukraine, Russia's new generation warfare is manifested in five component elements: political subversion, proxy sanctuary, intervention, coercive deterrence, and negotiated manipulation."
The invasion of Crimea was carried out quietly using the excuse of humanitarian intervention. In just three weeks and with less than 10,000 troops, Russia managed to dislodge Crimea from Ukraine and annex it, without a shot being fired, and the surrender of 190 Ukrainian bases to the Russians. The ground was prepared for this long before, via a multi-pronged "soft" approach including bribing local officials, paying off oligarchs, providing local separatists with intelligence information, investing heavily in local Ukrainian energy and industrial businesses, and the continual emission of pro-Russian propaganda through networks owned or funded by Russia, and a process of deception known as "maskirovka" ("camouflage" in Russian) to hide its operations.  It does the same thing domestically, with its vast control of local print media, but especially via television reporting, which is very dominant news source for most of the population of the country.
The strong governmental control over the media gives it a direct pipeline to influence public opinion in the country. Public support of the media is very high, with 88% believing that Europe and the United States are involved in an "information war" to manipulate Russian media, while the Russian government merely reports the facts.
Accompanying Russian aggression on the ground in Ukraine in 2014, Russia made direct nuclear threats against Ukraine, who had earlier given up their nuclear weapons under the 1994 Budapest Agreement in exchange for an explicit guarantees of its sovereignty from Moscow. Russia has continued to push the limits, operating close to NATO borders with dangerous over-flights, demonstrations of nuclear force, and hostile wargame exercises, causing anxiety among border NATO member countries as well as neutral countries.

Compared to other theories: New generation warfare doctrine differs from hybrid warfare in that it combines both low-end, clandestine operations with high-end direct superpower involvement.