There is a famous quote which I've seen phrased in several different ways.
"Armies prepare to fight their last war, rather than their next war"
"Generals prepare to fight their father's war"
I've seen this quote in many books and news stories; my question is who is credited with saying it first?
I don't think I understand the quote. Could you please explain?
At different periods in history, weapons and tactics have given significant advantages to forces on offense or defense. Over time, new innovations flip these advantages to the other side. When this has occurred militaries are often caught unprepared, focused on their last experience rather than alert to how emerging innovations are changing the properties of war.
At first, fortifications like castles and walled cities yielded advantages to defenders. It's why investments were made in hugely expensive fortifications and networks of castles throughout Europe.
Gunpowder and cannons flipped that and made castles and fortified cities irrelevant. For centuries with firearms, offenses held the advantage. A "Cult of offense" existed among military theorists which professed any effort made towards defense or protection of one's troops was a wasted effort. That troops on the offensive held such an advantage that all defensive efforts were counter productive. Would ultimately cause more casualties and contribute to defeat.
During WWI, the machine gun paired with barbed wire flipped that back again. The cult of offense still held by the generals dictated that hundreds of thousands of men would die in battles on the Western Front. As the defensive advantages demonstrated themselves. The Western Front ground to a stalemate. Major battles fought barely changed the fronts lines. Generals on both sides would order massive frontal assaults believing there troops on offense still held an advantage, while in reality machine guns behind fortified lines cut the offensive troops to ribbons.
In WWII, tanks and new tank tactics would again flip the advantages. Infantry behind massed armor destroyed or simply bypassed fortifications built at great expense by generals who had learned the now outdated lessons of early WWI. See Maginot Line.
The soldiers who best learn the lessons of the wars they fight advance to become the generals of future wars. They prepare to refight the wars they know. Only the nature of warfare changes. In modern times, these changes affecting war tactics have accelerated. Where once these changes occurred over centuries or even millennia, in more modern times they have occurred over decades.