The First World War is often identified as a turning point in men's hair length.

Prior to the war, both men and women commonly kept long hair, at least in western societies (and the Far East). This became problematic during the Great War, where armies encountered severe hygiene issues fighting in the trenches. Under the unsanitary conditions of the front, soldiers adopted short hair to mitigate the scourge of lice or fleas.

The men who fought in that war were numerous (65 million) and the 56 million survivors lived a further half century and had an overwhelming influence on those younger ... Men for the next half century had to measure themselves against the war veterans, to try to give themselves and others the impression that they too could have endured the trenches. That became the measure of manhood. And so the style of the veteran of the trenches - short hair, suntan, dangling cigarette, the taciturn, unfeeling, nonchalant post which does not flinch when someone a few yards away gets shot - becomes the male style of the twentieth century.

• Antony, Michael. The Masculine Century: A Heretical History of Our Time. iUniverse, 2008.

Due to the prestige attached to the veterans, their characteristic hair cut was retained and emulated as the armies demobilised. This was reinforced by the adoption of crew cuts for servicemen during WWII.

Obviously there's a great deal of generalisation going on when we speak in such terms as the hair length of entire societies. Some men, even today, kept their hair long. Meanwhile, many women also cut their hair short, especially during the inter-war years. The bob cut for instance became popularised after 1920.

This wasn't the first time people cut their hair short, though. As in anchient greece alexander the great's soldiers shaved their hair and beards so it couldn't be grabbed by the enemy. Also in the Industrial Revolution progressed in the 18th century, short hair became attractive to those working with machinery for safety reasons. Similarly, many women cut their hair short for the same reason, when they entered the war factories during the two World Wars.

In fact, typical hair lengths has been very variable throughout history. What you call "obvious identity signs" are not constant and has never been universally true in history. Although WW1 and WW2 may be said to have kicked off the current fashion trend, it should not be considered either permanent or unprecedented.