The 2018 Nuclear Posture Review concluded that the U.S. must deploy a small number of low-yield nuclear warheads on its Trident missiles to deter Russian first use of low-yield nuclear weapons for limited nuclear strikes in conventional warfare. It states, “Russia’s belief that limited nuclear first use, potentially including low-yield weapons, can provide such an advantage is based, in part, on Moscow’s perception that its greater number and variety of non-strategic nuclear systems provide a coercive advantage in crises and at lower levels of conflict. Recent Russian statements on this evolving nuclear weapons doctrine appear to lower the threshold for Moscow’s first-use of nuclear weapons. Russia demonstrates its perception of the advantage these systems provide through numerous exercises and statements. Correcting this mistaken Russian perception is a strategic imperative.” 
The threat of Russian first use involving limited nuclear strikes was recognized by the Obama administration. In October 2016, Secretary of Defense Ashton Carter stated that “…it’s a sobering fact that the most likely use of nuclear weapons is not the massive nuclear exchange of the classic Cold War-type, but rather the unwise resort to smaller but still unprecedentedly terrible attacks, for example, by Russia or North Korea to try to coerce a conventionally superior opponent to back off or abandon an ally during a crisis.”  As Secretary of Defense Jim Mattis has stated, “We want to make certain they recognize that we can respond in kind and that we don't have to go with the high yield weapon.” 
Domestic opposition to this NPR decision appears to be largely ideological and ignores the disparity in such weapons that now exists and Russia’s doctrine with regards to the first use of nuclear weapons. There is nothing new about low-yield warheads on ballistic missiles. In addition to Russia, the UK and France reportedly have low-yield ballistic missile warheads on their SLBMs. 
Russia reportedly has acquired low-yield, precision low-yield and low-collateral damage nuclear weapons. This was originally reported by distinguished Russian journalist Pavel Felgenhauer who wrote in 2002 that in April 1999 the Russian National Security Council approved a concept for developing and using “…non-strategic low- and flexible-yield battlefield weapons,” and that the yield of these precision weapons would be tens or hundreds of tons of TNT.  “Flexible yield” is clearly what we call variable yield or dial a yield. New Russian low-yield nuclear weapons are reported in the Russian press including in the state media. A declassified year 2000 CIA report observed, “Moscow’s military doctrine on the use of nuclear weapons has been evolving and probably has served as the justification for the development of very low-yield, high-precision nuclear weapons. The range of applications will ultimately be determined by Russia’s evolving nuclear doctrine, and could include artillery, air-to-air weapons, ABM weapons, anti-satellite weapons or multiple rocket launchers against tanks or massed troops.…”  In 2009, the bipartisan U.S. Strategic Commission report said Russia was developing “…low-yield tactical nuclear weapons including an earth penetrator.” 
Actual Russian deployment of strategic low-yield nuclear warheads (tens of tons to 200 tons yield) on Russian SLBMs (the Sineva and Bulava-30) has been reported in the state media (Sputnik News and Ria Novosti), and in the decidedly non-state media, the liberal Ekho Moskvy Radio (in an interview with hardline but well connected Russian journalist Colonel (ret.) Viktor Litovkin.).  Since, according to then-Defense Minister Sergei Ivanov, the Bulava-30 SLBM and RS-24/Yars ICBM use the same new warhead, it is nearly certain that the RS-24 also has low- yield options. 
Russian media have also reported that Russia has developed and deployed new low-yield tactical nuclear weapons. In 2004, Russian television displayed a new howitzer which it said: “…could be used to fire low-yield nuclear bombs.”  In 2013, Academician Yevgeniy Avrorin, a former Director of the Sarov nuclear weapons laboratory (the All-Russian Scientific-Research Institute), in an interview published by the Sarov laboratory, said the Russian 152-mm nuclear artillery shell with “a kiloton yield” has been “broadly deployed” throughout the Russian Army.  In 2009, Russia’s main official news agency ITAR-TASS (now called TASS) reported that, “The nuclear submarine Severodvinsk will be equipped with long-range cruise missiles that can potentially carry low-capacity tactical warheads.” The 2018 NPR report indicates that Russia has CRBM (Close Range Ballistic Missiles) which would have to have low-yield warheads because of their limited range. Dr. Philip Karber, President of the Potomac Foundation, has stated that roughly half of Russia’s 5,000 tactical nuclear weapons have been modernized with new sub-kiloton nuclear warheads for air-defense, torpedoes and cruise missiles. 
Russia is also reportedly developing advanced low-collateral damage designs. In 1999, Major General (ret.) Vladimir Belous discussed the development of “neutron artillery shells, mortar shells, and operational-tactical missile warheads.”  A declassified CIA report gives this some additional credibility, noting, “A number of articles [in the Russian press] suggest that Russia is developing low-yield warheads with enhanced radiation that could be used on high-precision non-strategic weapons systems.”  Moreover, in 2013, the Sarov nuclear weapons laboratory said that during the Cold War they had developed a peaceful nuclear explosive (PNE) device that was 99.85% based on fusion.  This is essentially a low-yield/low-collateral damage nuclear weapon. The only question would be its size and weight but, worst case, it certainly could be delivered by any strategic bomber which can deliver large and heavy weapons. Vice Admiral (ret.) Robert Monroe, former Director of the Defense Nuclear Agency, has recently stated that Russia is now 20 years ahead of the U.S. in such weapons.