Which in particular? That the concept of a balance of power is essentially meaningless needs no historical examples. Nobody has ever bothered to define what one actually is, for instance. That's a rather big stumbling block: how can anyone be so sure that the "balance is disrupted" if you don't demonstrate what it was in the first place? Invariably, different statesmen develop different concepts of what a "balance" is; shockingly, these all seem to favor the country they govern. So the United Kingdom, probably the only state to deploy "balance of power" reasoning before the twentieth century, defined a balance as, loosely, the various states of Europe being too weak and too internally divided to challenge Britain itself, with the unsaid corollary being that the United Kingdom continually grows stronger outside Europe, where apparently no balance is needed. A contemporary statesman from, say, France, would regard this definition as ridiculous. Instead, he would claim that a balance - if he suddenly decided to use this way of putting things - meant that France had no enemies on the Continent, while the UK was prevented from doing anything untoward by rivals overseas, e.g. the United States, Spain, Russia, or whomever. Take the case of Edward Grey, the British foreign minister in 1914. Grey believed that Germany was disrupting the balance of power, yes, and that the UK had to take a stand on the Continent in order to deal with it. It's unclear as to how Germany "imbalanced" things when the Central Powers were unquestionably weaker in every military regard compared to the Entente in 1914: manpower, organization, scale of mobilization, naval quality, financial resources, industrial production, size of battle fleet, number of potential allies to draw into the conflict...hence the rather understandable German view at the time, espoused by the Kaiser and Chancellor Bethmann Hollweg, that Germany was being "encircled" preparatory to being dismantled by war and revolution at the hands of the conniving Entente, and that Germany had to fight in 1914 because if they waited, Germany would have an even smaller chance of saving itself from the abyss.
Furthermore, it has been quite effectively demonstrated - and the more intelligent of the balance-of-power adherents fully admit this - that a balance, even if it were to exist, would neither work towards peace or towards war. Wars have happened because there was a balance and states wished to find a way past the stalemate; wars have happened because there was an imbalance and states wished to "redress" it, too, according to these "realists". And since, again, virtually nobody outside the UK subscribed to this theory for most of history, if a balance has any effect at all on the thinking of statesmen, it did an amazing vanishing act. That last bit is the kicker, at least for me. It's unquestionably true that statesmen do not necessarily view even the concept of a deterrent the same way, and it's further true that statesmen lead their countries into war despite low to no odds of success as they themselves acknowledge. Indeed, this was the case in Vienna in 1914. Why on Earth should they draw back from the abyss due to some abstract "balance" thought up by somebody in some other country's Foreign Office? The natural question to ask, then, would be why the concept of a balance matters at all.
Take Gangor's comment specifically. He claims that the balance of power in Europe was disrupted by the unification of Germany and that the First World War was the inevitable redressing of said balance. That comes out to forty-three years of imbalance with no general European war and a sizable increase in living standards across the continent, whereas a redressed balance led to further political turmoil, revolution, famine, military coups, economic depression, and eventually an even bigger war two decades later. One might tend to think that imbalanced power is a Good Thing for peace. Of course, he doesn't demonstrate that post-1871 Europe was "imbalanced" by the arrival of Germany, or how pre-1870 Europe wasn't "imbalanced". There is no inquiry into the intervening four decades between the Treaty of Frankfurt and the July Crisis as to possible 'shifts' of the balance there. And the problem of causation - the ever-present problem of causation - is never addressed. The outbreak of war is painted as a response to a Germany that apparently had gotten too big for its britches, never mind the actual cause of the war being a crisis of internal and external security for the Austro-Hungarian Empire. Apparently Austria-Hungary declared war on Serbia because Germany had gotten too powerful, and Russia declared war on Austria-Hungary for the same reason. Right.
