The United States maintains a policy of "no first use" of nuclear weapons, meaning that it will not use nuclear weapons in a first strike scenario, and only considers the use of nuclear weapons in response to a nuclear attack or other grave threat. The corresponding policy in Russia is "escalate to de-escalate," which suggests that it may be willing to use nuclear weapons in response to a conventional attack that threatens its nuclear capabilities. This policy is designed to deter potential aggressors from engaging in a conventional attack against Russia.
A nuclear umbrella is a term used to describe a security arrangement where a nuclear power promises to use its nuclear weapons to protect and defend its allies in the event of a nuclear attack. Essentially, it is a form of extended deterrence, where the threat of nuclear retaliation by a more powerful state serves to deter an attack against a weaker state or its allies. However, it's highly unlikely that the United States would use nuclear weapons on Russia in response to a nuclear attack on i.e. Ukraine or Sweden as such a move would risk a disastrous nuclear exchange between the two countries and potentially result in the total destruction of both United States and Russia.
If Russia perceives that its vital interests are threatened, it might feel compelled to use nuclear weapons even if that action is not intended to start a nuclear war. In the event of a nuclear attack by Russia on Ukraine the United States would reportedly respond with a range of conventional military options, diplomatic and economic measures, and potentially limited nuclear options, depending on the scale and nature of the attack.
"Just to give you a hypothetical, we would respond by leading a Nato – a collective – effort that would take out every Russian conventional force that we can see and identify on the battlefield in Ukraine and also in Crimea and every ship in the Black Sea." –David Petraeus, retired four-star army general and former director of CIA
Interference of West in Ukraine would lead to "such consequences that you have never encountered in your history." –Vladimir Putin
Also the technological developments in the West could potentially pose a threat to Russian nuclear capabilities and this is likely one of the factors that Russia considers in its nuclear policy. The combination of technological developments in the West and Russia's isolation from the West could potentially increase the risk of conflict and heighten tensions between the two sides. If Russia feels increasingly threatened by Western technological developments, it may be more inclined to rely on its nuclear capabilities as a means of deterrence, which could increase the risk of a nuclear exchange.
Russia's isolation from the West do limit the open channels of communication. Misunderstandings and misperceptions could potentially arise, leading to an escalation of tensions and slow development of tensions could suddenly turn extremely dangerous and the situation can spiral out of control. The use of propaganda or disinformation campaigns could fuel tensions and create an atmosphere of mistrust and hostility. Tensions can also be deliberately escalated for strategic reasons, such as to gain a military or political advantage. In such cases, one or both sides may take steps that increase the risk of a conflict, with the hope that the other side will back down or be deterred.
"We want to see Russia weakened to the degree that it can’t do the kinds of things that it has done in invading Ukraine." –Lloyd Austin, US Defense Secretary
"Why do we need a world if Russia is not in it?" –Dmitry Kiselyov, Russian state television
