Russia's war on Ukraine is getting more brutal, but 2 factors may keep Putin's finger off the nuclear button

• President Vladimir Putin has addled the West by setting Russia's nuclear forces on heightened alert.
• Knowing when those weapons will not be used is one of the most challenging tasks under the current circumstances, but there are two reasons to think Putin will think twice.

Russian President Vladimir Putin's announcement that he was setting his country's nuclear forces on heightened alert during his unprovoked attack on Ukraine has addled the West. This was no doubt part of Putin's purpose in making such a threat.

Presumably, the war in Ukraine reflects Putin's desire to alter international conditions, something that would be pointless if he intends to destroy the world in the process. Thus is the paradox of nuclear weapons, always ready, but (almost) never in use.

The march of technology has ensured that an increasing number of nations possess a growing range of weapons. Humanity has devoted a considerable portion of its collective ingenuity to fashioning an ever more diversified number of ways of eliminating other human beings.

In this context, one of the more important, and challenging tasks is to figure out under what circumstances certain weapons will not be used. In the case of nuclear weapons, this can determine national policy. Indeed, were it not for the plausibility of Putin's threat, the West might now be more actively and directly involved in the defense of Ukraine.

Military dogs that do not bark can now exceed the ones that do. Such is the case in the ongoing war in Ukraine, where many of the Russian army's most brutal weapons have yet to be utilized in battle, though this may be changing rapidly. Why not use a weapon of war, in a war? There are multiple reasons. I focus on just two here:

The first reason is that a weapon may be excessive. Either the weapon will not accomplish the task that political or military authorities desire, or it will do so too well. Nuking Ukraine would definitely end the war, nominally in Putin's favor, but what would be left would hardly be worth controlling. Putin presumably wants to pacify and control Ukraine, not terminate the existence of most of its people and poison its territory.

This is why initially Russian forces were reluctant to escalate to what British Defense Secretary Ben Wallace has described as "full tonto." Doing so wrecks much of what an invader may seek to capture, including the public's good will.

This does not mean an aggressor will refrain from later resorting to the very weapons that were initially shunned. The frustration stemming from unsuccessful initial measures will often result in resorting to other weapons, just as we now see Russian forces beefing up their numbers and resorting to more vicious weapons, including thermobaric rockets, in Ukraine.

Expecting to be welcomed with open arms, but instead received by armed citizens, Russia is now beginning to unleash a much more vigorous, and barbaric, form of mechanized warfare on the residents of Ukraine's cities.

The second rationale for not using a particular weapon is that it is escalatory. Use of a weapon may be expected to invite an unwanted response from others. We witness public discussions in the West about possible Russian retaliation for certain actions, such as deploying NATO "boots on the ground" in Ukraine.

The German public fretted about certification of the Nord Stream 2 pipeline, out of fear that Russia would cut off heat to Western Europe in the middle of winter. More recently, officials in Washington debated whether to unleash America's cyber weapons against Russia, delaying for now in part because of concern that Russia would reply in kind.

The core objective in any contest is to win. Any action governments take either advances or retards this goal, depending on whether an effort harms or inhibits an adversary more or less than oneself.

There is thus always a winner and a loser from any initiative in war. Either the use of a particular military or civilian tool helps us more than them, or the reverse. We steer our efforts towards the kinds of actions that favor us and harm opponents, in relative terms.

But the enemy also has agency; they will be trying to do the opposite. Ideally, governments would like to checkmate the enemy, taking an action for which there is no adequate response. Achieving this condition of so-called "escalation dominance" is both prized and relatively rare.

More often, the actions one takes prompt an enemy to shift the contest in turn in ways that favor them and harm one's own side.

This is what is happening in all domains in the midst of this war. In cyberspace, where both sides have the potential to harm the other through interconnected electronic linkages, there is more of a stalemate and mutual deterrence than many imagined would be the case.

As with nuclear weapons, western fears of Russia using these tools have prevented unleashing the dogs of cyberwar against Russian assets. And Russia for its part does not want to trigger a wider contest, one that will almost certainly reduce, not increase, Russian prospects for victory in Ukraine.

Thus, there is the heavy hint that Russia is ready to use the weapons not yet being exercised — cyber, nuclear and others — but also the clear implication that Russia is not eager, indeed willing, to use these capabilities unilaterally, preemptively, and without necessity, even in the context of its aggressive war against Ukraine.

Erik Gartzke is professor of political science and director of the Center for Peace and Security Studies (cPASS) at the University of California, San Diego.