The most visible instance of this would be during the French revolution. Before the Revolution, officers ranks would only be available to those that could afford it and more importantly, be able to prove four generations of nobility (which was standard for anyone wanting a position within Ancien Regime France). Some officers were interested in being good officers but others would often treat their regiment as a second source of income, stealing from the regimental coffers and giving the men the minimum.

When the Revolution occurred and Feudalism was abolished, these ranks were available to any whom were deemed competent but later in the Revolution (during the Terror), higher ranks were chosen democratically (which oddly enough worked out well a majority of the time) with the men voting for their officers.

As the Revolution continued and battles weeded out the good from the bad, competency was easy to establish. Individuals like Andre Massena and Napoleon Bonaparte were able to claim victory from near impossible odds, turning away the enemies of the Republic in 1796-97. As a result, the armies got better and had far more upward mobility than the armies of Britain, Prussia, Russia, or Austria.

In Chinese history, before 300BC, generally military appointments were entirely based on birth, nobility, and currying favor with the Imperial court.

But by the time the Warring States Period, mass conscription was the mainstay of the various factions. There was a noticeable uptick in the 'commoner' element in armies and the vast size of armies necessitated many more officers, especially mid level officers that managed things on the ground and in the field.

This was the beginning of appointing commoners to positions of real authority, responsibility, and chances for leadership.

While some of the nobility were against it, as they saw it as the start of the erosion of their monopoly on military power, it would have been impossible to manage such large armies without many reliable, competent men to administer the minutia of army logistics, record keeping, training, etc.

One of the major reasons for this shift was the departure from chariot based warfare where most of the warriors were upper class professionals and nobles. Instead, the various states during the Warring States Period instituted mandatory military service and raised huge armies of infantry, supported by even larger armies of camp attendants, cooks, smiths, laborers, etc.

As time went on, a resourceful and capable leader would be promoted based on merits to higher and higher positions, regardless of birthright and relations. This would eventually be formalized into a promotion system much later on and would result in the military as being one of the major ways of social advancement in Chinese society for hundreds of years.

Because of this large change, the commoners were no longer simply just a source of income and labor. They were now an essential part of any lords military strength.

To reflect this, administrators began to collect a formal tax so in times of war, the taxes, whether collected as currency or goods, could be quickly used to raise large armies, buying equipment, supplies and animals. As such, this was the first time that peasants were 'paid' for their work as it was.

This was because their contributions to the lord was not simply their labor but their products and services. And a formally administered tax meant that they were 'officially' allowed to keep part of the fruits of their labors in a structured system.

So during the Warring States Period, not only were the military ranks opened up, but civil administration ranks were opened up as well to those who were competent and able.

So, one could argue that because of the great transition from small scale to large scale warfare, social advancement opportunities were created in bulk and made available to any who were capable, allowing common people to participate in roles of authority in the military and local administrations.

Consequently, nobles had much of their monopoly on power reduced. That being said, there was still no doubt that nobility were the privileged class. Nobles were still the only ones who had influence in the Imperial court, owned land officially, collected taxes, raised and led armies, etc.

So for China, this happened a very very long time ago, to say the least! And it certainly has effects even today.

This tradition of social advancement through service to the state is deeply ingrained in Chinese culture and history. Even today, the cushiest and ideal jobs for many is civil service, ie working for the government in administrative jobs, military, law, etc.

Commoners were allowed into the Prussian officer corps as part of reforms started after Prussia's defeat by Napoleon at Jena-Auerstadt in 1806. The Prussian officers corps had previously been dominated by the Junker aristocrats, and the change led to a slow increase in commoners over a period of decades. There was a reactionary movement by some senior officers from the Junker class that led to policies that decreased the amount of commoners joining the officer corps, but it was temporary and never completely reversed the change.

The reforms overall were very successful in improving the Prussian military, as can be seen from their performance in the War of the Seventh Coalition, the Austro-Prussian War, and the Franco-Prussian War. The significance of allowing commoners to become officers is hard to evaluate because it occurred simultaneously with several other military reforms. The two main effects it had on the Prussian military were helping change the officer corps into a meritocracy and connecting the Prussian people to the army. Junker aristocrats continued to be the majority of officers, but they were now forced to compete against commoners in officer candidate entrance examinations. Along with more intellectual officer training, this change forced all officers, regardless of class, to perform to a higher standard.

The Prussian army was perceived by the Prussian people as the personal army of the king, which it essentially was. Opening the officer corps, along with the creation of the Landwehr (national militia) helped to form a bond between the Prussian people and the Prussian Army.