The most likely cause of a nuclear exchange between the west and Russia will be a case of mistaken intentions, like the Able Archer incident, or the Stanislav Petrov false alarm. If you believe that you are going to be the target of a nuclear first strike, the incentive to strike first yourself is extremely high. If you firmly believed that you would be under attack tomorrow, you would be compelled to strike today.
Or if you felt you were already under attack, you would only have minutes to decide whether or not to launch a massive counterattack. ICBMs can take 20-30 minutes to go halfway around the world, but a stealthy bomber strike or sub-based attack could potentially shorten that significantly. You would have literally minutes to decide whether the signs of an incoming attack were legitimate and how to respond. You would hardly have time to contact the appropriate authorities, verify your information, or take any other action at all. You might wake up the president in the middle of the night and tell him he has 3 minutes to decide on potentially ending civilization. Standing idle could lose your ability to launch an effective counterstrike, and there’s a strong incentive to launch as soon as you believe it’s likely you’re under attack. Launch on warning has more or less officially been the policy of the US and Russia for decades.
The tensions between Russia and the west make some sort of misunderstanding more likely. When hostilities are high, you are more likely to interpret ambiguous data as hostile. You’re looking for reasons to suspect enemy hostility or attack. People seem to think that the threat of nuclear annihilation vanished at the end of the cold war, but we’ve still got thousands of warheads ready to go at a few minutes notice. Tensions have been as low as they have been because we settled into a stable situation. But the west’s encroachment into Russia’s sphere of influence after the cold war has been threatning to them. Russia’s historical need for a buffer zone comes from a deeply embedded cultural fear of foreign invasion, since they’ve almost been destroyed that way many times throughout history. The US exploited the waning of Russia’s global influence after the fall of the Soviet Union by encroaching upon and influencing what has traditionally been a Russian buffer zone. And because of this, we’re now obligated to annihilate civilizations for the sake of places like Estonia or Latvia just as we would be for somewhere like France or Great Britain. We’ve taken on obligations and liabilities and increased tension for no real benefit other than that the people who were coming into power during the cold war got to stick it to those Ruskies after it ended. Imagine how we’d feel if we lost the cold war, and Russia was the world’s only superpower, and they convinced Mexico to join the Warsaw Pact. Now that Putin is in power and Russia is regaining some of its military power, economic power, and international influence, they’re pushing back against these encroachments and we face the highest likelihood of war that we’ve faced in 25 years.
The development of ballistic missile defense systems and arms reduction treaties also increases the likelihood of a nuclear exchange. It may sound counterintuitive, but imagine if there were two hostile powers who weren’t at war but tensions were high. Each of them has one nuclear bomb, and has a good idea where the other side stores their nuke. They would both be compelled to race to launch at each other in the hopes that they could take out the other side’s nuke. Whereas if each side had a thousand nukes with survivable delivery systems, there would be an extreme disincentive for both sides to engage in any sort of strike, fearing that counterattack. MAD works, and it works best if both sides have large arsenals, capable delivery platforms, and survivable platforms for second strike options. It seems paradoxical but the more heavily armed the the two sides are, the stronger the disincentive to ever go to war in the first place, even if a war would be more destructive in the unlikely event that it happened.
The more likely your first strike is to disable the enemy’s ability to strike you back, the more incentivized you are in making such a strike. Having few warheads, poor delivery platforms, or platforms with low survivability makes an exchange more likely for that reason. So on the opposite side, having many warheads with survivable platforms makes an exchange less likely. We’ve been reducing the number of total warheads and active warheads since the end of the cold war, which creates a situation in which a counterforce strike becomes more viable. The fewer nukes each side has, the more winnable (or managable) a nuclear war seems.
If one side developed an effective ballistic missile shield, it would have the ability to strike at its enemies with relative impunity, which greatly upsets the balance of power. As such, if that power were in the process of developing and deploying that effective missile shield, the other side would have an incentive to attempt to strike before it was ready, since they would otherwise potentially lose the ability to strike at that power at all, leaving them in an extremely vulnerable position. Fortunately along these lines the development of anti-ballistic missiles is extremely difficult and as such the systems on either side are only effective - if at all - against a limited arsenal. But as they improve they continue to destabilize the situation. There’s a reason we created an anti-ballistic missile treaty in the first place. Just like bans on space-borne weaponry, it had a stabilizing effect on the situation and lowered tensions. But pulling out of it is another move that increases tension between Russia and the west.
It’s extremely unlikely either side would simply decide one day to attack the others with nukes - the cost on both sides is simply too high. But as tensions raise, each side becomes more paranoid and more convinced that they’re the ones that are going to be under attack. I think it’s extremely likely that if such an exchange occured, it would be because one side or the other felt like they were in imminent danger, rather than a deliberate plan on their part to be the aggressor. But we’re on a path to reigniting those tensions so that one false step or one misunderstanding could lead to the greatest tragedy the human race has ever seen.