The current equilibrium between the world powers is partly due to the limitations of military technology. For instance, when the United States took over Iraq and Afghanistan, the military casualties and economic costs were a significant fraction of the total available resources of the United States. That invasion wasn’t profitable, and most possible territory grabs have the same problem. The United States can’t invade nuclear armed nations, and vice versa.
This equilibrium may not be eternal.
An actual hermetic defense against nuclear attack would require patrolling every last mile of border with flying and ground robot drones. You’d need enough ABMs and defense satellites to defeat ICBMs with countermeasures, you’d need to scan every last piece of material entering or leaving the country with X-rays and NMR to detect fissionables.
In short, this kind of defense grid means you’d need an enormous industrial advantage - maybe 100:1 or so, over the rest of the world combined.
So this won’t be happening anytime soon. However, hypothetically, what if a nation did gain such a capability? Perhaps it develops self-replicating robotic systems or self replicating molecular assemblers, and after a few years of exponential growth can outproduce the rest of the planet combined.
This means from the perspective of a military leader, you’d have a temporary advantage (since whatever tech gave you the industrial advantage could eventually be copied), and the chance to eliminate all your enemies permanently.
You could in theory create drone aircraft, drone ground robots, and other weapons by the millions. These things would use a mixture of artificial intelligence and remote human operators.
You’d kill their leaders and soldiers until they surrendered unconditionally, and turn every remaining nation into a panopticon police state where the survivors can be watched from cradle to grave for threatening activity. (most of your infantry would be robotic, to make the manpower demands reasonable)
This may sound evil, but keep in mind that every nuclear armed nation on earth, even your “allies”, has had the option to murder your people by the millions since the 1960s or so. You’d offer them the choice to surrender their atomic arsenals and submit to annexation peacefully, and if they chose to fight, you’d fight to the finish.
On to the question : If you’re tasked with ensuring the freedom and security of your people, and you have a chance to eliminate the threats to your people forever at minimal risk to yourselves, don’t your duties compel you to take advantage of the opportunity?